Introduction

Ultra/Luminous infrared galaxies (U/LIRGS) are extraordinary systems that exhibit some of the most extreme star formation in the local universe. The intense star formation is believed to be triggered by the merger of two gas-rich galaxies but the mechanism is not yet well understood. High-resolution carbon monoxide (CO) observations show that the majority of molecular gas is concentrated near the nuclear regions of the systems. Since molecular gas is the fuel for current and future star formation, understanding the physical conditions and distribution of the warm, dense gas in the local U/LIRGS will help in better understanding the details of star formation in the most luminous galaxies and, of course, the history of galaxy formation and evolution.