Proteasome is a highly organized protease complex comprising a catalytic 20S core particle (CP) and two 19S regulatory particles (RP), which together form the 26S structure. The 26S proteasome is responsible for the degradation of most ubiquitylated proteins through a multistep process involving recognition of the polyubiquitin chain, unfolding of the substrate, and translocation of the substrate into the active site in the cavity of the CP. Recent studies have shed light on various aspects of the complex functions of the 26S proteasome. In addition, the recent identification of various proteasome-dedicated chaperones indicates that the assembly pathways of the RP and CP are multistep processes. In this review, we summarize recent advances in the understanding of the proteasome structure, function, and assembly.