The present feature article provides an overview on the use of state-of-the-art mass spectrometry techniques such as matrix assisted laser desorption and ionization time of flight (MALDI-TOF) mass spectrometry as well as electrospray ionization mass spectrometry (ESI-MS) for probing the mechanism of free radical polymerization processes. The article features representative examples of the application of mass spectrometry techniques to conventional free radical polymerization, nitroxide mediated polymerization (NMP), atom transfer radical polymerization (ATRP), reversible addition fragmentation chain transfer (RAFT) as well as catalytic chain transfer (CCT) processes. (C) 2004 Elsevier Ltd. All rights reserved.