The ability to assay in vivo biologic processes non-invasively and quantitatively makes molecular imaging a particularly attractive tool for clinical trials of new drugs and for clinical cancer practice. This review highlights the emerging application of molecular imaging to cancer drug testing and clinical cancer treatment. The potential roles that imaging can play in the approach to cancer drug trials and clinical treatment are first highlighted, including applications to early drug testing, Phase II and III clinical trials, and clinical practice. The use of molecular imaging to select cancer therapy is then discussed, citing examples where molecular imaging can be used to measure the expression of therapeutic targets and factors mediating therapeutic resistance. The use of imaging to measure early pharmacodynamic changes and subsequent response to cancer treatment is then reviewed. Finally, the need for standardization and reproducible quantitative imaging analysis is reviewed.