The chapter reviews the various effects of crowding on mitochondrial structure and function. Data illustrate that, as a rule, the concentration of macromolecules in the mitochondrial matrix and inner membrane is at least as high as in other parts of the cytoplasm, while the intermembrane space is sparse. The effect of crowding on mitochondrial shape, the role of disordered protein domains, and the role of changes in crowding during mitochondrial swelling are discussed. Because of the excluded volume in the matrix, real changes in ion and metabolite concentrations under swelling are substantially higher than believed, and the inner membrane is highly curved at least partly due to crowding of the matrix. The high concentration of integral proteins in the mitochondrial inner membrane leads to enrichment with non-bilayer lipid cardiolipin to compensate for their induced membrane deformations. Also crowding may be one of the stimuli for the formation of enzyme supercomplexes. All reviewed data suggest that the structure of mitochondria is adapted exactly to the conditions of high crowding, and crowding itself is one of the key factors in the regulation of mitochondrial structure and function, the role of which is significantly underestimated in the scientific literature.