diff --git a/src/core/nanoev.mli b/src/core/nanoev.mli
index 213b9de..d0a698f 100644
--- a/src/core/nanoev.mli
+++ b/src/core/nanoev.mli
@@ -1,4 +1,16 @@
-(** Nano event loop *)
+(** Nano event loop.
+
+    {1 Summary}
+
+    Nanoev is a small abstraction over non-blocking IO event loops offered by
+    most operating systems (e.g kqueue on BSD/macOS, poll on posix systems,
+    epoll on linux, completion ports on windows).
+
+    The idea is that a [Nanoev.t] encapsulates a single event loop in a reusable
+    form, and can be used in higher-level concurrency packages or to write async
+    IO code directly. See {!Nanoev_unix} for a [Unix.select]-based
+    implementation and {!Nanoev_posix} for a [poll]-based (better)
+    implementation. *)
 
 type t
 
@@ -29,31 +41,46 @@ module Impl : sig
     run_after_s: 'a 'b. 'st -> float -> 'a -> 'b -> ('a -> 'b -> unit) -> unit;
     step: 'st -> unit;
   }
+  (** A nanoev event loop provides a set of operations on a hidden state ['st].
+  *)
 
   val build : 'a ops -> 'a -> t
+  (** Build a [Nanoev.t] from operations and hidden state. *)
 end
 
 val clear : t -> unit
-(** Reset the state *)
+(** Reset the state, similar to creating a new event loop from scratch *)
 
 val wakeup_from_outside : t -> unit
+(** Wakeup a sleeping event loop from the outside (another thread, a signal
+    handler, etc.). This must be thread-safe. *)
 
 val step : t -> unit
-(** Run one step of the event loop until something happens *)
+(** Run one step of the event loop until something happens. This can potentially
+    block the caller for an interdeterminate duration. *)
 
 val close : t -> Unix.file_descr -> unit
-(** Close the file descriptor and clean it up *)
+(** Close the file descriptor and clean it up inside the event loop. Callbacks
+    registered on this FD will be called with [~closed:true]. *)
 
 val max_fds : t -> int
-(** Maximum number of file descriptors that can be observed at once. *)
+(** Maximum number of file descriptors that can be observed at once. This
+    depends on the syscall underlying the event loop, as well as other system
+    limits (see [ulimit -n] to change this limit on linux, for example). *)
 
 val on_readable :
   t -> Unix.file_descr -> 'a -> 'b -> (closed:bool -> 'a -> 'b -> unit) -> unit
+(** [on_readable nanoev fd x y f] registers [f x y] to be called when [fd]
+    becomes readable. An additional [closed] parameter is passed to [f] to
+    inform it of whether [fd] is {i known} to be closed. *)
 
 val on_writable :
   t -> Unix.file_descr -> 'a -> 'b -> (closed:bool -> 'a -> 'b -> unit) -> unit
+(** Same as {!on_readable} but calls [f] when the FD is writable. *)
 
 val run_after_s : t -> float -> 'a -> 'b -> ('a -> 'b -> unit) -> unit
+(** [run_after_s nanoev [duration] x y f] registers [f x y] to be called after
+    [duration] seconds have elapsed. *)
 
 (**/**)
 
diff --git a/src/unix/nanoev_unix.mli b/src/unix/nanoev_unix.mli
index 5aba925..4bbeffc 100644
--- a/src/unix/nanoev_unix.mli
+++ b/src/unix/nanoev_unix.mli
@@ -5,3 +5,5 @@ include module type of struct
 end
 
 val create : unit -> t
+(** Create a [Nanoev.t] based on [Unix.select]. This is fairly limited and only
+    works in processes that have fewer than 1024 FDs open in total. *)