diff --git a/.gitignore b/.gitignore
index 49e14ccc8d..0b199c7e56 100644
--- a/.gitignore
+++ b/.gitignore
@@ -10,3 +10,4 @@ venv*/
 .python-version
 build/
 dist/
+.vscode/
diff --git a/docs/advanced.md b/docs/advanced.md
index 1b0ecee7c5..d1ca5bd38d 100644
--- a/docs/advanced.md
+++ b/docs/advanced.md
@@ -1057,3 +1057,39 @@ Which we can use in the same way:
 >>> response.json()
 {"text": "Hello, world!"}
 ```
+
+
+## Custom JSON library support
+
+By default `httpx` uses built-in python library `json` to decode and encode
+JSON data.
+It is possible to use some other, higher performance library or add some custom
+encoder options.
+
+You can make `dumps` return pretty-printed JSON by default.
+```python
+import httpx
+import json
+
+def _my_dumps(obj, **kwargs):
+    return json.dumps(obj,
+        ensure_ascii=kwargs.pop('ensure_ascii', False),
+        indent=kwargs.pop('indent', 4),
+        sort_keys=kwargs.pop('sort_keys', True),
+        **kwargs
+    )
+
+with httpx.Client(json_encoder=_my_dumps) as client:
+    response = client.post('http://httpbin.org/anything', json={'Hello': 'World!', '🙂': '👋'})
+    print(response.json())
+```
+
+Or use another library for JSON support.
+```python
+import httpx
+import orjson
+
+with httpx.Client(json_encoder=orjson.dumps, json_decoder=orjson.loads) as client:
+    response = client.post('http://httpbin.org/anything', json={'Hello': 'World!', '🙂': '👋'})
+    print(response.json())
+```
diff --git a/httpx/_client.py b/httpx/_client.py
index d15c004530..6eb9909f2d 100644
--- a/httpx/_client.py
+++ b/httpx/_client.py
@@ -1,5 +1,6 @@
 import datetime
-import enum
+import functools
+import json
 import typing
 import warnings
 from types import TracebackType
@@ -90,6 +91,8 @@ def __init__(
         event_hooks: typing.Dict[str, typing.List[typing.Callable]] = None,
         base_url: URLTypes = "",
         trust_env: bool = True,
+        json_encoder: typing.Callable = json.dumps,
+        json_decoder: typing.Callable = json.loads,
     ):
         event_hooks = {} if event_hooks is None else event_hooks
 
@@ -108,6 +111,8 @@ def __init__(
         self._trust_env = trust_env
         self._netrc = NetRCInfo()
         self._state = ClientState.UNOPENED
+        self._json_encoder = json_encoder
+        self._json_decoder = json_decoder
 
     @property
     def is_closed(self) -> bool:
@@ -317,6 +322,7 @@ def build_request(
             params=params,
             headers=headers,
             cookies=cookies,
+            json_encoder=self._json_encoder,
         )
 
     def _merge_url(self, url: URLTypes) -> URL:
@@ -411,7 +417,12 @@ def _build_redirect_request(self, request: Request, response: Response) -> Reque
         stream = self._redirect_stream(request, method)
         cookies = Cookies(self.cookies)
         return Request(
-            method=method, url=url, headers=headers, cookies=cookies, stream=stream
+            method=method,
+            url=url,
+            headers=headers,
+            cookies=cookies,
+            stream=stream,
+            json_encoder=self._json_encoder,
         )
 
     def _redirect_method(self, request: Request, response: Response) -> str:
@@ -548,6 +559,8 @@ class Client(BaseClient):
     rather than sending actual network requests.
     * **trust_env** - *(optional)* Enables or disables usage of environment
     variables for configuration.
+    * **json_encoder** - *(optional)* A custom function to encode request data to JSON.
+    * **json_decoder** - *(optional)* A custom function to decode response data from JSON.
     """
 
     def __init__(
@@ -570,6 +583,8 @@ def __init__(
         transport: httpcore.SyncHTTPTransport = None,
         app: typing.Callable = None,
         trust_env: bool = True,
+        json_encoder: typing.Callable = json.dumps,
+        json_decoder: typing.Callable = json.loads,
     ):
         super().__init__(
             auth=auth,
@@ -581,6 +596,8 @@ def __init__(
             event_hooks=event_hooks,
             base_url=base_url,
             trust_env=trust_env,
+            json_encoder=json_encoder,
+            json_decoder=json_decoder,
         )
 
         if http2:
@@ -878,6 +895,8 @@ def on_close(response: Response) -> None:
             ext=ext,
             request=request,
             on_close=on_close,
+            json_encoder=self._json_encoder,
+            json_decoder=self._json_decoder,
         )
 
         self.cookies.extract_cookies(response)
@@ -1185,6 +1204,8 @@ class AsyncClient(BaseClient):
     rather than sending actual network requests.
     * **trust_env** - *(optional)* Enables or disables usage of environment
     variables for configuration.
+    * **json_encoder** - *(optional)* A custom function to encode request data to JSON.
+    * **json_decoder** - *(optional)* A custom function to decode response data from JSON.
     """
 
     def __init__(
@@ -1207,6 +1228,8 @@ def __init__(
         transport: httpcore.AsyncHTTPTransport = None,
         app: typing.Callable = None,
         trust_env: bool = True,
+        json_encoder: typing.Callable = json.dumps,
+        json_decoder: typing.Callable = json.loads,
     ):
         super().__init__(
             auth=auth,
@@ -1218,6 +1241,8 @@ def __init__(
             event_hooks=event_hooks,
             base_url=base_url,
             trust_env=trust_env,
+            json_encoder=json_encoder,
+            json_decoder=json_decoder,
         )
 
         if http2:
@@ -1519,6 +1544,8 @@ async def on_close(response: Response) -> None:
             ext=ext,
             request=request,
             on_close=on_close,
+            json_encoder=self._json_encoder,
+            json_decoder=self._json_decoder,
         )
 
         self.cookies.extract_cookies(response)
diff --git a/httpx/_content.py b/httpx/_content.py
index bf402c9e29..8bbd9f8ecb 100644
--- a/httpx/_content.py
+++ b/httpx/_content.py
@@ -1,5 +1,6 @@
 import inspect
-from json import dumps as json_dumps
+import json
+import typing
 from typing import (
     Any,
     AsyncIterable,
@@ -137,8 +138,12 @@ def encode_html(html: str) -> Tuple[Dict[str, str], ByteStream]:
     return headers, PlainByteStream(body)
 
 
-def encode_json(json: Any) -> Tuple[Dict[str, str], ByteStream]:
-    body = json_dumps(json).encode("utf-8")
+def encode_json(
+    json: Any, encoder: typing.Callable = json.dumps
+) -> Tuple[Dict[str, str], ByteStream]:
+    body: Union[bytes, str] = encoder(json)
+    if isinstance(body, str):
+        body = body.encode("utf-8")
     content_length = str(len(body))
     content_type = "application/json"
     headers = {"Content-Length": content_length, "Content-Type": content_type}
@@ -151,6 +156,7 @@ def encode_request(
     files: RequestFiles = None,
     json: Any = None,
     boundary: bytes = None,
+    json_encoder: typing.Callable = json.dumps,
 ) -> Tuple[Dict[str, str], ByteStream]:
     """
     Handles encoding the given `content`, `data`, `files`, and `json`,
@@ -173,7 +179,7 @@ def encode_request(
     elif data:
         return encode_urlencoded_data(data)
     elif json is not None:
-        return encode_json(json)
+        return encode_json(json, encoder=json_encoder)
 
     return {}, PlainByteStream(b"")
 
@@ -183,6 +189,7 @@ def encode_response(
     text: str = None,
     html: str = None,
     json: Any = None,
+    json_encoder: typing.Callable = json.dumps,
 ) -> Tuple[Dict[str, str], ByteStream]:
     """
     Handles encoding the given `content`, returning a two-tuple of
@@ -195,6 +202,6 @@ def encode_response(
     elif html is not None:
         return encode_html(html)
     elif json is not None:
-        return encode_json(json)
+        return encode_json(json, encoder=json_encoder)
 
     return {}, PlainByteStream(b"")
diff --git a/httpx/_models.py b/httpx/_models.py
index c981c740bf..56d3b760ed 100644
--- a/httpx/_models.py
+++ b/httpx/_models.py
@@ -2,7 +2,7 @@
 import contextlib
 import datetime
 import email.message
-import json as jsonlib
+import json
 import typing
 import urllib.request
 from collections.abc import MutableMapping
@@ -793,6 +793,7 @@ def __init__(
         files: RequestFiles = None,
         json: typing.Any = None,
         stream: ByteStream = None,
+        json_encoder: typing.Callable = json.dumps,
     ):
         if isinstance(method, bytes):
             self.method = method.decode("ascii").upper()
@@ -820,7 +821,9 @@ def __init__(
             self.stream = stream
             self._prepare({})
         else:
-            headers, stream = encode_request(content, data, files, json)
+            headers, stream = encode_request(
+                content, data, files, json, json_encoder=json_encoder
+            )
             self._prepare(headers)
             self.stream = stream
 
@@ -903,6 +906,8 @@ def __init__(
         ext: dict = None,
         history: typing.List["Response"] = None,
         on_close: typing.Callable = None,
+        json_encoder: typing.Callable = json.dumps,
+        json_decoder: typing.Callable = json.loads,
     ):
         self.status_code = status_code
         self.headers = Headers(headers)
@@ -936,7 +941,9 @@ def __init__(
             # from the transport API.
             self.stream = stream
         else:
-            headers, stream = encode_response(content, text, html, json)
+            headers, stream = encode_response(
+                content, text, html, json, json_encoder=json_encoder
+            )
             self._prepare(headers)
             self.stream = stream
             if content is None or isinstance(content, (bytes, str)):
@@ -944,6 +951,7 @@ def __init__(
                 self.read()
 
         self._num_bytes_downloaded = 0
+        self._json_decoder = json_decoder
 
     def _prepare(self, default_headers: typing.Dict[str, str]) -> None:
         for key, value in default_headers.items():
@@ -1106,14 +1114,17 @@ def raise_for_status(self) -> None:
             raise HTTPStatusError(message, request=request, response=self)
 
     def json(self, **kwargs: typing.Any) -> typing.Any:
+        """
+        For available `kwargs` see `json.loads` definition.
+        """
         if self.charset_encoding is None and self.content and len(self.content) > 3:
             encoding = guess_json_utf(self.content)
             if encoding is not None:
                 try:
-                    return jsonlib.loads(self.content.decode(encoding), **kwargs)
+                    return self._json_decoder(self.content.decode(encoding), **kwargs)
                 except UnicodeDecodeError:
                     pass
-        return jsonlib.loads(self.text, **kwargs)
+        return self._json_decoder(self.text, **kwargs)
 
     @property
     def cookies(self) -> "Cookies":
diff --git a/tests/client/test_async_client.py b/tests/client/test_async_client.py
index 44ff90fe51..6eb4117c9b 100644
--- a/tests/client/test_async_client.py
+++ b/tests/client/test_async_client.py
@@ -1,4 +1,6 @@
+import json
 from datetime import timedelta
+from decimal import Decimal
 
 import httpcore
 import pytest
@@ -242,3 +244,20 @@ async def test_deleting_unclosed_async_client_causes_warning():
     await client.get("http://example.com")
     with pytest.warns(UserWarning):
         del client
+
+
+@pytest.mark.usefixtures("async_environment")
+async def test_post_json_overriden_decoder(server):
+    def _my_loads(s, **kwargs):
+        return json.loads(s, parse_float=lambda v: Decimal(v), **kwargs)
+
+    url = server.url
+    async with httpx.AsyncClient(json_decoder=_my_loads) as client:
+        response = await client.post(
+            url.copy_with(path="/echo_body"),
+            json={"text": "Hello, world!", "decimal": 0.12345},
+        )
+    assert response.status_code == 200
+    data = response.json()
+
+    assert isinstance(data["decimal"], Decimal)
diff --git a/tests/client/test_client.py b/tests/client/test_client.py
index a41f4232fb..1f2b75a5ae 100644
--- a/tests/client/test_client.py
+++ b/tests/client/test_client.py
@@ -1,4 +1,6 @@
+import json
 from datetime import timedelta
+from decimal import Decimal
 
 import httpcore
 import pytest
@@ -300,3 +302,19 @@ def test_raw_client_header():
         ["User-Agent", f"python-httpx/{httpx.__version__}"],
         ["Example-Header", "example-value"],
     ]
+
+
+def test_post_json_overriden_decoder(server):
+    def _my_loads(s, **kwargs):
+        return json.loads(s, parse_float=lambda v: Decimal(v), **kwargs)
+
+    url = server.url
+    with httpx.Client(json_decoder=_my_loads) as client:
+        response = client.post(
+            url.copy_with(path="/echo_body"),
+            json={"text": "Hello, world!", "decimal": 0.12345},
+        )
+    assert response.status_code == 200
+    data = response.json()
+
+    assert isinstance(data["decimal"], Decimal)
diff --git a/tests/test_content.py b/tests/test_content.py
index 384f9f2287..3c335435f3 100644
--- a/tests/test_content.py
+++ b/tests/test_content.py
@@ -1,4 +1,5 @@
 import io
+import json
 import typing
 
 import pytest
@@ -357,3 +358,34 @@ async def hello_world():
 def test_response_invalid_argument():
     with pytest.raises(TypeError):
         encode_response(123)  # type: ignore
+
+
+@pytest.mark.asyncio
+async def test_json_content_overriden_encoder():
+    def _my_dumps(obj, **kwargs):
+        return json.dumps(obj, ensure_ascii=False, sort_keys=True, **kwargs).encode(
+            "utf-8"
+        )
+
+    data = {
+        "こんにちは": "世界",
+        "ওহে": "বিশ্ব!",
+        "Привет": "мир!",
+        "Hello": "world!",
+    }
+    headers, stream = encode_request(json=data, json_encoder=_my_dumps)
+
+    assert isinstance(stream, typing.Iterable)
+    assert isinstance(stream, typing.AsyncIterable)
+
+    sync_content = b"".join([part for part in stream])
+    async_content = b"".join([part async for part in stream])
+
+    result = _my_dumps(data)
+
+    assert headers == {
+        "Content-Length": str(len(result)),
+        "Content-Type": "application/json",
+    }
+    assert sync_content == result
+    assert async_content == result