The canonical Wnt/β-catenin signaling pathway plays critical roles in both embryonic development and tumorigenesis. Central to the pathway is the turnover of β-catenin, a protein that functions in both cell adhesion and transcription. In the absence of a Wnt signal, free cytosolic β -catenin is phosphorylated by a large protein complex called the “ β-catenin destruction complex ” that targets β-catenin for degradation by an ubiquitin ligase/proteasome system. In the presence of a Wnt signal, the binding of Wnt to its receptor Frizzled and co-receptor LRP leads to the inhibition of β-catenin phosphorylation in the β-catenin destruction complex through an unknown mechanism. Inhibition of the β-catenin destruction complex leads to the accumulation of nuclear β-catenin, which in turn forms a complex with Tcf and BCL9. Recent studies have provided important clues regarding the molecular mechanism of the β-catenin destruction complex as well as an explanation for how β-catenin switches between its roles in cell adhesion and transcription.