Solid tumours differ from normal tissue in a number of important ways. Rapid growth and metabolism of tumours often leads to altered physiology including changes in vasculature and perfusion which can in turn induce fluctuations in the supply of nutrients such as oxygen. Reduced tissue oxygenation, known as hypoxia, has been linked to radio- and chemotherapeutic resistance and is often associated with a poor prognosis for disease-free survival in cancer patients. Identification of tumour hypoxia by using non-invasive imaging techniques such as positron emission tomography (PET) is vital for clinicians to provide effective treatment. However, physiological imaging modalities such as PET require the design and administration of radioactive imaging agents which specifically target tumours. This book provides a background to the basic chemical concepts and methods used in the rational design of tumour-selective radiopharmaceuticals with particular emphasis on the mechanisms by which copper-based imaging agents target tumour hypoxia.