With a focus on fine structures, the interiors of cells and their architectures come to life in the higher organization of tissues. The authors provide insight into the complex cellular interior with its diverse functional spaces and dynamic processes that are essential for fulfilling cellular tasks within the tissue through this atlas of functional ultrastructure. The special aim is to make the reader aware that these are the structures where fundamental cellular life processes and disease-causing mechanisms occur. Using electron microscopy images, supplemented by immunoelectron microscopy and detailed descriptions, the relationships between structures and functions within the cells are unraveled, details are highlighted, and the organization within the cellular associations of tissues is presented.