Elongation is becoming increasingly recognized as a critical step in eukaryotic 
transcriptional regulation. Although traditional genetic and biochemical studies 
have identified major players of transcriptional elongation, our understanding 
of the importance and roles of these factors is evolving rapidly through the 
recent advances in genome-wide and single-molecule technologies. Here, we focus 
on how elongation can modulate the transcriptional outcome through the 
rate-liming step of RNA polymerase II (Pol II) pausing near promoters and how 
the participating factors were identified. Among the factors we describe are the 
pausing factors--NELF (negative elongation factor) and DSIF (DRB 
sensitivity-inducing factor)--and P-TEFb (positive elongation factor b), which 
is the key player in pause release. We also describe the high-resolution view of 
Pol II pausing and propose nonexclusive models for how pausing is achieved. We 
then discuss Pol II elongation through the bodies of genes and the roles of FACT 
and SPT6, factors that allow Pol II to move through nucleosomes.