The seven mammalian members of the signal transducer and activator of transcription (STAT) family share a common core structure which reflects their shared mechanism of activation, dimerization, and DNA binding. By contrast, the STAT C termini containing the sequences required for transcriptional activation are much less homologous, suggesting different ways by which individual STATs activate their target genes. This paper describes several important discoveries linked to mechanistic aspects of STAT transcription factor function. These include regulated serine phosphorylation of the transactivating domain, promoter-dependent interactions of STATs with each other, or of STATs with other transcription factors, and with transcriptional co-activators. The basis, background, and implications of these molecular events will be summarized and discussed.