Proteasome inhibition is a modern and surprisingly successful approach how to cancer treatment. Bortezomib (Velcade®) is a first-in-class proteasome inhibitor and has been approved for first-line treatment of multiple myeloma and second-line treatment of mantle cell lymphoma. There have been almost 200 clinical trials with bortezomib, both as a single agent and in combination with other drugs, for various cancers, as listed in the US National Cancer Institute database. However, bortezomib's mechanism of action remains elusive despite enormous progress in our knowledge of the cell biology of the ubiquitin-proteasome system (UPS) and bortezomib-induced signaling events in cancer cells. This review maps a rapidly growing and open body of research in both areas.