How many does it take to tango? -- Critical mass phenomena in the analysis of chemotactic aggregation

Critical mass phenomena belong to the most striking subtleties in Keller-Segel type aggregation dynamics. Starting from classical findings concerned with two-dimensional problems, the presentation aims at describing some recent developments which confirm crucial roles of total population sizes in a large number of contexts. In particular, the presence $L^1$ thresholds in some higher-dimensional problems is discussed, as well as the occurrence of certain secondary critical mass phenomena.