The availability of specific imaging probes is the nuclear fuel for molecular imaging by positron emission tomography and single-photon emission computed tomography. These two radiotracer-based imaging modalities represent the prototype methods for noninvasive depiction and quantification of biochemical processes, allowing a functional characterization of tumor biology. A variety of powerful radiolabeled probes--tracers--are already established in the routine clinical management of human disease and others are currently subject to clinical assessment. Emerging from investigations of the genomic and proteomic signatures of cancer cells, an increasing number of promising targets are being identified, including receptors, enzymes, transporters, and antigens. Corresponding probes for these newly identified targets need to be developed and transferred into the clinical setting. Starting with a brief summary of the characteristics and prerequisites for a "good tracer," an overview of tracer concepts, target selection, and development strategies is given. The influence of the imaging concepts on tracer development is also discussed.