Living cells, including red blood cells, insulin-producing cells, and stem cells, offer great potential for treating various diseases but face significant practical challenges. For example, the demand for blood transfusions is challenged by mismatched blood types, immune responses, and donation shortages, driving research toward the development of universal blood solutions. Similarly, transplantation of insulin-producing cells offers an alternative to frequent injections but is limited by low insulin secretion, high cell death, inflammation, and immune activation, leading to graft failure. Stem cells hold promise for various disease treatments but face significant challenges such as low survival rate, undesired differentiation, limited targeting, and lack of specific cell interactions. These challenges in blood transfusion, islet transplantation, and stem cell therapies highlight the need for single-cell or cell aggregate nanoencapsulation strategies. This chapter discusses cell encapsulation methods, summarizing strategies such as electrostatic layer-by-layer assembly, supramolecular engineering, covalent reactions, hydrogel-based coatings, and bioorthogonal glycoengineering. This chapter highlights applications such as protecting insulin-producing cells for diabetes, camouflaging red blood cells for safe transfusion, and resurfacing stem cell surfaces for tissue engineering through single-cell or cell aggregate nanoencapsulation. This chapter further outlines future research directions in single-cell and cell aggregate nanoencapsulation for biomedical applications.