Caspases, which constitute a family of cysteine proteases, are highly conserved in multicellular organisms and function as a central player in apoptosis. The detection of apoptosis is intrinsically difficult because dying cells are rapidly removed from tissues by phagocytosis. Thus, the development of a method for detecting caspase activation is critical for the in vivo study of apoptosis. In this chapter, we describe a genetically encoded fluorescent probe for live imaging of caspase activation.