Nanobodies are the small (15 kDa) and stable single-domain fragments harboring the full antigen-binding capacity of camelid heavy chain–only antibodies. Nanobodies are exquisite chaperones for crystallizing membrane proteins, multiprotein assemblies, transient conformational states and intrinsically disordered proteins. Nanobodies can also be used for other applications in structural biology. Domain-specific Nanobodies have been used in single-particle electron microscopy (EM) to track these domains in particle projections. Because Nanobodies can be functionally expressed as intrabodies in eukaryotic cells, these single-domain antibodies can also be used to track their targets inside a living cell.