X error

Hi, Below is the end of a log output. Apparently it is looking for X. However I have no idea which X it is refering, and where to find it. Thank you for any help/suggestion you can offer. checking for pkg-config... /opt/local/bin/pkg-config checking pkg-config is at least version 0.9.0... yes checking for X_CFLAGS... checking for X_LIBS... checking for X... no configure: error: X is required, but it was either disabled or not found. Warning: the following items did not execute (for xrender): org.macports.activate org.macports.configure org.macports.build org.macports.destroot org.macports.install