In the ubiquitylation system, E3 ubiquitin ligases play a key role in determining substrate specificity and catalyzing the transfer of ubiquitin from E2 enzymes to the substrate. Growing evidence has shown that E3 ubiquitin ligases are involved in cancer development and progression. The RING-type and HECT-type E3 ligases are the classically categorized groups of E3 ubiquitin ligases, and more of these enzymes are being shown to be potential targets for cancer therapy. The recently classified RBR E3 ligases catalyze the transfer of ubiquitin by a RING/HECT hybrid-like mechanism. Notably, these ligases are also emphasized as important potential candidates for targets of cancer treatment drugs. The present review provides an overview of the RING-, HECT- and RBR-type E3 ligases, and discusses their roles in cancer and cancer therapy.