Ubiquitin-mediated proteolysis plays a major role in a variety of cellular functions, including cell metabolism, cell cycle progression, cellular response to DNA damage, and programmed cell death. In most cases, the crucial regulators involved in the control of these diverse cellular functions are modified by specific E3 ubiquitin ligases through the attachment of multiple ubiquitin molecules, a signal that triggers the subsequent destruction by the 26S proteasome complex. Recent studies revealed that the proteasomal degradation pathway regulates the cellular apoptosis process on multiple levels. Thus, a better understanding of the molecular mechanisms that underlie the ubiquitination and destruction of these specific regulators of apoptosis will provide us with insight on how apoptosis is properly controlled in normal cells and how tumor cells evade the apoptosis pathways. This chapter provides an overview of the common methods used to examine whether a target protein is ubiquitinated, as well as the protocols to examine how a putative E3 ligase controls the destruction of the target protein.