In this first part, the physical and technical aspects of positron emission tomography (PET), single-photon emission computed tomography (SPECT), computed tomography (CT), and their integrated forms are discussed. The emphasis is on clinical imaging, and therefore theory is presented in such a way that clinicians can grasp the concepts without difficulty. The goal here is not to give full technical details but to educate the reader on some of the basic aspects that will allow proper understanding of these imaging methods. The references provide adequate reading material for individuals who want to understand the methods in more detail. The integrated imaging modalities PET-CT and SPECT-CT have not been around for very long, and thus a final verdict on several technical aspects has not yet been reached. In addition, the constituent imaging systems are undergoing rapid development. CT in particular has seen an unprecedented development in its capabilities over the last 5 years. Nevertheless, some insights into how to use the integrated techniques have stabilized over the last years and are unlikely to change. The key is thus to conserve the relevant old and adopt the exciting new!