remote X

I'm not sure where/why I'm failing but... I have /etc/ssh/sshd_conf entery of: X11Forwarding yes (which is the default config, as is everything else) When I try to run X-based programs from another PC's ssh session, I get errors like: cannot connect to X server Can't open display: I'm probably missing some little variable somewhere, but heavens know if I can find it.