Activation of the phosphoinositide-3-kinase (PI3K) signaling pathway is frequently found in common human cancers, brought about by oncogenic receptor tyrosine kinases (RTKs) acting upstream, PTEN loss, or activating mutations of PI3K itself. Recent studies have delineated distinct but overlapping functions in cell signaling and tumorigenesis for p110α and p110β, the two major catalytic subunits of PI3K expressed in the tissues of origin for the common tumor types. In most cell types studied, p110α carries the majority of the PI3K signal in classic RTK signal transduction, while p110β responds to GPCRs. Both p110α and p110β function in cellular transformation induced by alterations in components of PI3K pathway. Specifically, p110α is essential for the signaling and growth of tumors driven by PIK3CA mutations and/or oncogenic RTKs/Ras, whereas p110β is the major isoform in mediating PTEN-deficient tumorigenesis. While pan-PI3K inhibitors are currently being tested in the clinic, p110 isoform-specific inhibition holds promise as a therapeutic strategy.