Molecular and functional imaging aims to assess oncologic therapy response by integrating molecular and functional tumor biology in order to assess therapeutic efficacy and improve patient outcome. Most oncologic processes reflect heterogeneous disease both functionally and morphologically. Further, clonal proliferations of cells may evolve with time becoming resistant to specific therapies. It is important to identify those cancer patients who derive benefit from therapy, such that expensive, toxic, or futile treatment is avoided in those who will not respond. The ultimate goal is to offer the right treatment to the right patient over time. Molecular and functional imaging either using positron emission tomography (PET) or gamma cameras often through hybrid scanners that also include computed tomography (CT) and/or magnetic resonance imaging (MRI) are sensitive techniques with a major role in the precision medicine algorithm of oncology patients. These modalities provide insight prior to, during, and following therapy. Further, they often serve as a biomarker of tumoral heterogeneity helping to direct the selection of appropriate treatment, and detect early response to therapy. Also, molecular and functional imaging is a powerful prognostic biomarker in oncology that can suggest patient outcome based on treatment response.