Eukaryotes have three distinct RNA polymerases that catalyze transcription of nuclear genes. RNA polymerase II is responsible for transcribing nuclear genes encoding the messenger RNAs and several small nuclear RNAs. Like RNA polymerases I and III, pol II cannot recognize its target promoter directly and initiate transcription without accessory factors. Instead, this large multisubunit enzyme relies on both general transcription factors and transcriptional activators and coactivators to regulate transcription from class II promoters. At the center of this process is TFIID, a 700-kD complex composed of the TATA box binding protein (TBP) and a set of phylogenetically conserved, polymerase-specific TBP-associated factors or TAFIIS. Together, TBP and the TAFIIS direct assembly of the transcription machinery and play critical regulatory roles in eukaryotic gene expression.