In Arabidopsis thaliana more than 1300 genes (~5% of the proteome) encode components of the ubiquitin/26S proteasome pathway. Approximately 90% of these genes encode subunits of the E3 ubiquitin ligases, which confer substrate specificity to the ubiquitin/26S proteasome pathway. The plant E3 ubiquitin ligases comprise a large and diverse family of proteins or protein complexes containing a RING-finger, U-box domain or a HECT domain. Within the past several years, considerable progress has been achieved in understanding the role of protein degradation via the ubiquitin-proteasome pathway in hormone responses, self-incompatibility, photomorphogenesis and pathogen defenses.