diff --git a/README.md b/README.md
index d5d2148713..9bf6026d17 100644
--- a/README.md
+++ b/README.md
@@ -97,10 +97,12 @@ Install with pip:
 $ pip install httpx
 ```
 
-Or, to include the optional HTTP/2 support, use:
+There are also a number of optional dependancies.
+
+For example to include asyncio and HTTP/2 support, use:
 
 ```shell
-$ pip install httpx[http2]
+$ pip install 'httpx[asyncio,http2]'
 ```
 
 HTTPX requires Python 3.8+.
@@ -129,15 +131,17 @@ The HTTPX project relies on these excellent libraries:
   * `h11` - HTTP/1.1 support.
 * `certifi` - SSL certificates.
 * `idna` - Internationalized domain name support.
-* `sniffio` - Async library autodetection.
 
 As well as these optional installs:
 
-* `h2` - HTTP/2 support. *(Optional, with `httpx[http2]`)*
-* `socksio` - SOCKS proxy support. *(Optional, with `httpx[socks]`)*
-* `rich` - Rich terminal support. *(Optional, with `httpx[cli]`)*
-* `click` - Command line client support. *(Optional, with `httpx[cli]`)*
-* `brotli` or `brotlicffi` - Decoding for "brotli" compressed responses. *(Optional, with `httpx[brotli]`)*
+* `anyio` - Async support for `asyncio`. *(Optional, with `httpx['asyncio']`)*
+* `trio` - Async support for `trio`. *(Optional, with `httpx['trio']`)*
+* `h2` - HTTP/2 support. *(Optional, with `httpx['http2']`)*
+* `socksio` - SOCKS proxy support. *(Optional, with `httpx['socks']`)*
+* `click` - Command line client support. *(Optional, with `httpx['cli']`)*
+* `rich` - Command line client support. *(Optional, with `httpx['cli']`)*
+* `pygments` - Command line client support. *(Optional, with `httpx['cli']`)*
+* `brotli` or `brotlicffi` - Decoding for "brotli" compressed responses. *(Optional, with `httpx['brotli']`)*
 * `zstandard` - Decoding for "zstd" compressed responses. *(Optional, with `httpx[zstd]`)*
 
 A huge amount of credit is due to `requests` for the API layout that
diff --git a/docs/async.md b/docs/async.md
index 089d783191..4b62d85e72 100644
--- a/docs/async.md
+++ b/docs/async.md
@@ -10,6 +10,24 @@ long-lived network connections such as WebSockets.
 If you're working with an async web framework then you'll also want to use an
 async client for sending outgoing HTTP requests.
 
+## Enabling Async support
+
+To enable async support you'll need to install some additional dependencies:
+
+If you're using Python's [standard `asyncio` support](https://docs.python.org/3/library/asyncio.html) then:
+
+```shell
+$ pip install "httpx[asyncio]"
+```
+
+Or, if you're working with the [`trio` third party package](https://trio.readthedocs.io/en/stable/):
+
+```shell
+$ pip install httpx['trio']
+```
+
+We highly recommend `trio` for async support. The `trio` project [pioneered the principles of structured concurrency](https://en.wikipedia.org/wiki/Structured_concurrency), and has a more carefully constrained API against which to work from.
+
 ## Making Async requests
 
 To make asynchronous requests, you'll need an `AsyncClient`.
diff --git a/docs/http2.md b/docs/http2.md
index 3cab09d912..a865b55a26 100644
--- a/docs/http2.md
+++ b/docs/http2.md
@@ -28,7 +28,7 @@ trying out our HTTP/2 support. You can do so by first making sure to install
 the optional HTTP/2 dependencies...
 
 ```shell
-$ pip install httpx[http2]
+$ pip install 'httpx[http2]'
 ```
 
 And then instantiating a client with HTTP/2 support enabled:
diff --git a/httpx/_transports/asgi.py b/httpx/_transports/asgi.py
index 8578d4aeff..17ad2b0a14 100644
--- a/httpx/_transports/asgi.py
+++ b/httpx/_transports/asgi.py
@@ -2,8 +2,6 @@
 
 import typing
 
-import sniffio
-
 from .._models import Request, Response
 from .._types import AsyncByteStream
 from .base import AsyncBaseTransport
@@ -28,7 +26,9 @@
 __all__ = ["ASGITransport"]
 
 
-def create_event() -> Event:
+def create_event() -> "Event":
+    import sniffio
+
     if sniffio.current_async_library() == "trio":
         import trio
 
diff --git a/httpx/_transports/default.py b/httpx/_transports/default.py
index 33db416dd1..3ebf210249 100644
--- a/httpx/_transports/default.py
+++ b/httpx/_transports/default.py
@@ -277,6 +277,14 @@ def __init__(
         retries: int = 0,
         socket_options: typing.Iterable[SOCKET_OPTION] | None = None,
     ) -> None:
+        try:
+            import sniffio  # noqa: F401
+        except ImportError:  # pragma: nocover
+            raise RuntimeError(
+                "Using httpx in async mode, but neither "
+                "httpx['asyncio'] or asyncio['trio'] is installed."
+            )
+
         ssl_context = create_ssl_context(verify=verify, cert=cert, trust_env=trust_env)
         proxy = Proxy(url=proxy) if isinstance(proxy, (str, URL)) else proxy
 
diff --git a/httpx/_utils.py b/httpx/_utils.py
index 46a4d63b1d..0160d61dba 100644
--- a/httpx/_utils.py
+++ b/httpx/_utils.py
@@ -11,8 +11,6 @@
 from pathlib import Path
 from urllib.request import getproxies
 
-import sniffio
-
 from ._types import PrimitiveData
 
 if typing.TYPE_CHECKING:  # pragma: no cover
@@ -291,29 +289,18 @@ def peek_filelike_length(stream: typing.Any) -> int | None:
 
 
 class Timer:
-    async def _get_time(self) -> float:
-        library = sniffio.current_async_library()
-        if library == "trio":
-            import trio
-
-            return trio.current_time()
-        else:
-            import asyncio
-
-            return asyncio.get_event_loop().time()
-
     def sync_start(self) -> None:
         self.started = time.perf_counter()
 
     async def async_start(self) -> None:
-        self.started = await self._get_time()
+        self.started = time.perf_counter()
 
     def sync_elapsed(self) -> float:
         now = time.perf_counter()
         return now - self.started
 
     async def async_elapsed(self) -> float:
-        now = await self._get_time()
+        now = time.perf_counter()
         return now - self.started
 
 
diff --git a/pyproject.toml b/pyproject.toml
index c4c188052e..484989a1d8 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -29,10 +29,8 @@ classifiers = [
 ]
 dependencies = [
     "certifi",
-    "httpcore==1.*",
-    "anyio",
+    "httpcore>=1.0.0,<2.0.0",
     "idna",
-    "sniffio",
 ]
 dynamic = ["readme", "version"]
 
@@ -47,10 +45,16 @@ cli = [
     "rich>=10,<14",
 ]
 http2 = [
-    "h2>=3,<5",
+    "httpcore[http2]",
 ]
 socks = [
-    "socksio==1.*",
+    "httpcore[socks]",
+]
+asyncio = [
+    "httpcore[asyncio]"
+]
+trio = [
+    "httpcore[trio]"
 ]
 zstd = [
   "zstandard>=0.18.0",
diff --git a/requirements.txt b/requirements.txt
index 5aab03767c..bb2ddd202c 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -2,7 +2,7 @@
 # On the other hand, we're not pinning package dependencies, because our tests
 # needs to pass with the latest version of the packages.
 # Reference: https://github.com/encode/httpx/pull/1721#discussion_r661241588
--e .[brotli,cli,http2,socks,zstd]
+-e .[asyncio,trio,brotli,cli,http2,socks,zstd]
 
 # Optional charset auto-detection
 # Used in our test cases
@@ -24,6 +24,5 @@ mypy==1.11.2
 pytest==8.3.2
 ruff==0.6.3
 trio==0.26.2
-trio-typing==0.10.0
 trustme==1.1.0
 uvicorn==0.30.6