Large-scale Spitzer surveys of the Galactic plane have yielded a new tracer of massive young stellar objects (MYSOs) that are actively accreting and driving outflows: extended emission in the IRAC 4.5 μm band, believed to arise from shocked molecular gas. The GLIMPSE catalog of extended 4.5 μm sources (called EGOs, Extended Green Objects, for the common coding of 3-color IRAC images) is a unique sample, well-suited for studying the initial stages of massive star formation and establishing an observation-based MYSO evolutionary sequence. In this proceeding, we present results from extensive recent high-resolution Submillimeter Array (SMA) and Very Large Array (VLA) observations of EGOs at cm-mm wavelengths, including maser, molecular line, and continuum studies. These observations often reveal (proto)clusters that exhibit chemical and evolutionary diversity.
