Detection and observation of primary tumor growth and metastasis in living subjects is an important task in clinical and basic cancer research. Recently several approaches and techniques emerged which offer a huge variety of options with respect to the specific objectives and questions of a given study. Recent developments in the field of in vivo imaging not only allow the assessment of anatomic information but also functional processes with cellular resolution and molecular sensitivity. This chapter will provide an overview of the most common imaging techniques which are currently available for the detection and observation of metastasizing tumor cells. General capacities, advantages, limitations and drawbacks will be discussed. These techniques include computed tomography (CT), molecular resonance imaging (MRI), positron emission tomography (PET), single photon emission computed tomography (SPECT), fluorescence imaging (FI), and bioluminescent imaging (BLI). The objective is to provide the cancer researcher with information that will help solve the dilemma of how best to apply the latest imaging tools for studying biological questions in the context of the living body.