Cancer is one of the most mutable diseases known, exhibiting a superfluity and heterogeneity of molecular pathways that impart an almost chimerical nature to it. Exploiting these pathways for patient therapy demands an understanding of the physiology of tumors from the molecular to the systemic level. To this end, multiparametric functional and molecular imaging play a vital role in not only tracking delivery and efficacy of therapy, but also in discovering novel therapeutic targets. The plethora of available magnetic resonance (MR) contrast mechanisms, in conjunction with its superior dynamic functional range, bestow on magnetic resonance imaging (MRI) the potential to be a formidable tool in the noninvasive, in vivo, multilevel assessment of tumor physiology. This chapter begins with a description of the aberrant pathophysiology of tumors, including a description of tumor angiogenesis and how MRI affords us a window into such processes. Following a discussion of endogenous and exogenous contrast, a specific example of measuring a tumor's vascular parameters with a macromolecular contrast agent is considered. This is followed by a description of revolutionary developments in the molecular imaging of tumors with MRI and complementary modalities.