How to obtain a list of directories within a directory, like list.files(), but instead "list.dirs()"

I am able to use list.files() to obtain a list of files in a given directory, but if I want to get a list of directories, how would I do this? Is it somehow right in front of me as an option within list.files()?

Also, I'm using Windows, so if the answer is to shell out to some Linux/unix command, that won't work for me.

.NET for example has a Directory.GetFiles() method, and a separate Directory.GetDirectories() method, so I figured R would have an analogous pair.