I will discuss the formation of young massive clusters (YMCs) in our Galaxy. YMCs are the most massive bound stellar groups forming in the Galactic disk today. We have probed their formation with observations of dust emission throughout the Galaxy. While we now have a reasonable census of how many are forming and where, it remains unclear what conditions lead to the formation of a bound cluster with M>104 solar masses. In order to try to understand their formation and environment, we are examining the nearest forming YMC in W51. I will describe the use of formaldehyde as a tracer of gas density and show the contrast in volume density it has revealed in the different components of the W51 molecular cloud. Higher resolution observations have revealed the massive cores in the center of the protocluster along with a population of compact continuum sources more widely distributed. Our observations hint that two modes of massive star formation, both a relatively isolated and a highly clustered, interactive mode, coexist within the same complex.