The advances in the design and development of novel molecular imaging agents continue to occur globally at an accelerating rate. Radiotracers to probe and interrogate biochemical, cellular, and genetic processes involved in a myriad of disease states are being identified and assessed in both in vitro and in vivo systems. Over the past two decades, interest in developing radiolabeled peptides for imaging and treatment of cancers has been rapidly increasing. Technological advances in molecular biology, combinatorial chemistry, and peptide biochemistry are being used as a molecular basis to characterize receptor expression and to develop cognate receptor–avid radiolabeled peptide conjugates. The majority of the radiolabeled peptide conjugates discussed in this summary are analogues of naturally occurring regulatory peptides that act on multiple cellular targets in the human body. These regulatory peptides control and modulate important functions of cells and organs. Their action is often mediated through specific membrane-bound receptors and most belong to the class of G protein–coupled receptors.