A family of cysteine proteases, the caspases, plays a central role in the initiation and execution phases of apoptosis. Upon activation, these enzymes cleave specific substrates and thereby mediate many of the typical biochemical and morphological changes in apoptotic cells, such as cell shrinkage, chromatin condensation, DNA fragmentation and plasma membrane blebbing. Hence, the detection of activated caspases can be used as a biochemical marker for apoptosis. Here we review a set of methods available for characterizing and quantifying the activation of caspases, including immunoblotting, cleavage of synthetic substrates, affinity labeling and confocal microscopy. Each method is described in general terms and the advantages and disadvantages of each technique are discussed.