Although recent progress has been made in the treatment of mantle cell lymphoma (MCL) the majority of patients experience relapse and ultimately die of their disease. The translocation t(11;14) is a prerequisite for the diagnosis of MCL and results in overexpression of cyclin D1. Its protein translation is controlled by mTOR, a key element of the PI3K/Akt pathway, and mTOR constitutes an attractive therapeutic target. Temsirolimus, a specific inhibitor of mTOR, has been evaluated in two Phase II trials in patients with relapsed MCL, and promising response rates up to 40% were found. Subsequently, a randomized Phase III trial was initiated, in which superiority in remission induction and progression-free survival could be demonstrated for a regimen of temsirolimus 175 mg for 3 weeks, followed by a 75-mg weekly application in comparison with established agents. This adds temsirolimus to the therapeutic armamentarium for the treatment of MCL. Further developments target combination therapy in MCL and other lymphoid neoplasms.