I started using tmux recently after a) I was informed that GNU screen is basically an outdated POS, and b) an excellent book on the subject was published. I'm glad to have made the switch, as tmux is a wonderful improvement over screen. However, on OSX, I found that the pbcopy and pbpaste commands (among other things) would no longer work from within a tmux session.

After a bit of searching, I found this post that explains how to make things right again.

TL;DR:

brew install reattach-to-user-namespace

# Reattach to user namespace so that clipboard integration with OSX works again
set-option -g default-command "reattach-to-user-namespace -l zsh"

Obviously, replace zsh above with whatever you use for your shell.