SPECT and PET are nuclear tomographic imaging modalities that visualize functional information based on the accumulation of radioactive tracer molecules. However, SPECT and PET lack anatomical information, which has motivated their combination with an anatomical imaging modality such as CT or MRI. This chapter begins with an overview of the fundamental physics of SPECT and PET, followed by a presentation of the respective detector technologies, including detection requirements, principles, and different detector concepts. The reader is subsequently provided with an introduction to hybrid imaging concepts, before a dedicated section presents the challenges that arise when hybridizing SPECT or PET with MRI, namely, mutual distortions of the different electromagnetic fields in MRI on the nuclear imaging system and vice versa, and MR-based attenuation correction. The chapter then gives an overview about current hybrid imaging systems of both clinical and preclinical kinds and recent technological developments such as total body PET-CT, before describing the role of artificial intelligence in hybrid imaging. Finally, future developments in hybrid SPECT and PET technology are discussed.