The surface science community lost one of its bright young stars, Gregory E. Poirier, who passed away in September at the age of 39. During Greg's brief career at NIST he made many noteworthy contributions in the areas of surface science and chemical sensing, however, it was his STM studies of alkanethiol self-assembled monolayers (SAMs) on gold that earned him the greatest recognition. Greg's STM images were some of the first to reveal the structural complexity and phase behavior that governs the 2-D world of alkanethiols on gold. Through meticulous and rigorous interpretation of these images he unlocked many structural details of SAMs and gained an understanding of the molecular forces that govern the assembly of SAMs. This talk will review some of the highlights of Greg's STM studies including the rich variety of molecular-scale crystal structures of alkanethiol SAMs, their associated defect structures, and the development of a general mechanism for SAM formation.