Bortezomib is a proteasome inhibitor. It targets the ubiquitin-proteasome pathway with subsequent inhibition of the degradation of proteins involved in cell cycle regulation and cancer cell survival. The best known molecular mechanism concerns the inhibition of IκB breakdown and the related stabilization of NFκB, thus preventing its translocation to the nucleus for the activation of downstream pathways. Bortezomib is the only drug in this class which has been approved for clinical use. It has shown an efficient antitumor effect in a phase III clinical trial (APEX) involving relapsed multiple myeloma patients. Response rate, time to progression and overall survival have been improved in patients treated with bortezomib and dexamethasone compared to dexamethasone alone. These results have induced several researchers to suggest preclinical and clinical studies for the application of bortezomib in solid tumors. Preclinical data have proved useful in the identification of several of the biological processes implicated, including cell cycle arrest at the G2/M phase, upregulation of p21, apoptosis regulation, microvessel density reduction, overcoming chemotherapy resistance. The clinical results obtained so far with the use of bortezomib in patients with solid malignancies are still not sufficient for the introduction of the drug into clinical practice. Furthermore, the results obtained with the use of bortezomib combined with cytotoxic drugs have not proved any more satisfactory than those obtained with bortezomib used as a single agent. Other preclinical studies are required in order to reach a clearer understanding of the relevance of bortezomib in the therapy of solid tumors.