Mammalian ubiquitous mu- and m-calpains, as well as their Drosophila homologs, Calpain A and Calpain B, are Ca2+-activated cytoplasmic proteases that act by limited proteolysis of target proteins. Calpains are thought to be part of many cellular signaling pathways. These enzymes, however, require such high Ca2+ concentration for half-maximal activation in vitro, [Ca2+](0.5), that hardly ever occurs in intact cells. This major dilemma has pervaded the literature on calpains for decades. In this paper several considerations are put forward that challenge the orthodox view and envisage mechanisms that may govern calpain action in vivo. The "unphysiologically" high Ca2+ demand for activation may turn out to be an evolutionarily adjusted safety device. (C) 2004 Elsevier Inc. All rights reserved.