diff --git a/unwind/request.py b/unwind/request.py
index b3aed67..057cf7b 100644
--- a/unwind/request.py
+++ b/unwind/request.py
@@ -4,7 +4,7 @@ import logging
 import os
 import tempfile
 from collections import deque
-from contextlib import contextmanager
+from contextlib import asynccontextmanager, contextmanager
 from dataclasses import dataclass, field
 from functools import wraps
 from hashlib import md5
@@ -24,8 +24,10 @@ if config.debug and config.cachedir:
     config.cachedir.mkdir(exist_ok=True)
 
 
+_shared_asession = None
 _shared_session = None
 
+_ASession_T = httpx.AsyncClient
 _Session_T = httpx.Client
 _Response_T = httpx.Response
 
@@ -58,6 +60,32 @@ def _Session() -> _Session_T:
     return s
 
 
+@asynccontextmanager
+async def asession():
+    """Return the shared request session.
+
+    The session is shared by all request functions and provides cookie
+    persistence and connection pooling.
+    Opening the session before making a request allows you to set headers
+    or change the retry behavior.
+    """
+    global _shared_asession
+
+    if _shared_asession:
+        yield _shared_asession
+        return
+
+    _shared_asession = _ASession_T()
+    _shared_asession.headers[
+        "user-agent"
+    ] = "Mozilla/5.0 Gecko/20100101 unwind/20230203"
+    try:
+        async with _shared_asession:
+            yield _shared_asession
+    finally:
+        _shared_asession = None
+
+
 def _throttle(
     times: int, per_seconds: float, jitter: Callable[[], float] | None = None
 ) -> Callable[[Callable], Callable]:
@@ -298,3 +326,127 @@ def download(
         # Fix file attributes.
         if resp_lastmod is not None:
             os.utime(file_path, (resp_lastmod, resp_lastmod))
+
+
+async def adownload(
+    url: str,
+    *,
+    to_path: Path | str | None = None,
+    replace_existing: bool | None = None,
+    only_if_newer: bool = False,
+    timeout: float | None = None,
+    chunk_callback=None,
+    response_callback=None,
+) -> bytes | None:
+    """Download a file.
+
+    If `to_path` is `None` return the remote content, otherwise write the
+    content to the given file path.
+    Existing files will not be overwritten unless `replace_existing` is set.
+    Setting `only_if_newer` will check if the remote file is newer than the
+    local file, otherwise the download will be aborted.
+    """
+    if replace_existing is None:
+        replace_existing = only_if_newer
+
+    file_exists = None
+    if to_path is not None:
+        to_path = Path(to_path)
+
+        file_exists = to_path.exists() and to_path.stat().st_size
+        if file_exists and not replace_existing:
+            raise FileExistsError(23, "Would replace existing file", str(to_path))
+
+    async with asession() as s:
+        headers = {}
+        if file_exists and only_if_newer:
+            assert to_path
+            file_lastmod = _last_modified_from_file(to_path)
+            headers["if-modified-since"] = email.utils.formatdate(
+                file_lastmod, usegmt=True
+            )
+
+        req = s.build_request(method="GET", url=url, headers=headers, timeout=timeout)
+
+        log.debug("⚡️ Loading %s (%a) ...", req.url, dict(req.headers))
+        resp = await s.send(req, follow_redirects=True, stream=True)
+
+        try:
+            if response_callback is not None:
+                try:
+                    response_callback(resp)
+                except:
+                    log.exception("🐛 Error in response callback.")
+
+            log.debug(
+                "☕️ %s -> status: %s; headers: %a",
+                req.url,
+                resp.status_code,
+                dict(resp.headers),
+            )
+
+            if resp.status_code == httpx.codes.NOT_MODIFIED:
+                log.debug(
+                    "✋ Remote file has not changed, skipping download: %s -> %a",
+                    req.url,
+                    to_path,
+                )
+                return
+
+            resp.raise_for_status()
+
+            if to_path is None:
+                await resp.aread()  # Download the response stream to allow `resp.content` access.
+                return resp.content
+
+            resp_lastmod = _last_modified_from_response(resp)
+
+            # Check Last-Modified in case the server ignored If-Modified-Since.
+            # XXX also check Content-Length?
+            if file_exists and only_if_newer and resp_lastmod is not None:
+                assert file_lastmod
+
+                if resp_lastmod <= file_lastmod:
+                    log.debug("✋ Local file is newer, skipping download: %a", req.url)
+                    return
+
+            # Create intermediate directories if necessary.
+            download_dir = to_path.parent
+            download_dir.mkdir(parents=True, exist_ok=True)
+
+            # Write content to temp file.
+            tempdir = download_dir
+            tempfd, tempfile_path = tempfile.mkstemp(
+                dir=tempdir, prefix=f".download-{to_path.name}."
+            )
+            one_mb = 2**20
+            chunk_size = 8 * one_mb
+            try:
+                log.debug("💾 Writing to temp file %s ...", tempfile_path)
+                async for chunk in resp.aiter_bytes(chunk_size):
+                    os.write(tempfd, chunk)
+                    if chunk_callback:
+                        try:
+                            chunk_callback(chunk)
+                        except:
+                            log.exception("🐛 Error in chunk callback.")
+            finally:
+                os.close(tempfd)
+
+            # Move downloaded file to destination.
+            if to_path.exists():
+                log.debug("💾 Replacing existing file: %s", to_path)
+            else:
+                log.debug("💾 Move to destination: %s", to_path)
+            if replace_existing:
+                Path(tempfile_path).replace(to_path)
+            else:
+                Path(tempfile_path).rename(to_path)
+
+            # Fix file attributes.
+            if resp_lastmod is not None:
+                log.debug("💾 Adjusting file timestamp: %s (%s)", to_path, resp_lastmod)
+                os.utime(to_path, (resp_lastmod, resp_lastmod))
+
+        finally:
+            await resp.aclose()