Installing R with Homebrew

As of 2017 / Brew 1.3.2 @ macOS Sierra 10.12.6 all you have to do is:

$ brew install r

You don't even need to tap homebrew/science since r is now a part of core formulae for the Homebrew (homebrew-core).

It will also install all dependencies automatically:

==> Installing dependencies for r: gmp, mpfr, libmpc, isl, gcc

There are two additional options you might want to know:

--with-java
Build with java support
--with-openblas
Build with openblas support

As of 2017, it's just brew install r. See @Andrew's answer below.

As of 2014 (using an Yosemite), the method is the following:

brew tap homebrew/science
brew install Caskroom/cask/xquartz
brew install r

The gcc package (will be installed automatically as a required dependency) in the homebrew/science tap already contains the latest fortran compiler (gfortran), and most of all: the whole package is precompiled so it saves you a lot of compilation time.

This answer will also work for El Capitan and Mac OS Sierra.

In case you don't have XCode Command Line Tools (CLT), run from terminal:

xcode-select --install

I used this tutorial to install R on my mac, and it had me install xquartz and a fortran complier (gfortran) as well.

My suggestion would be to brew untap homebrew/science and then brew tap homebrew/science and try again, also, make sure you don't have any errors when you run brew doctor

Hope this helps