We use cryo-EM to study the structures of large self assembling macromolecular complexes, including enzyme polymers that regulate cellular metabolism, cytoskeletal structures that organize the intracellular space, and force-generating protein machines. In each of these systems protein-protein interactions drive creation of large cellular structures with unique functional properties. We complement our structural work with functional studies using a broad array of biophysical, biochemical, and cell biological approaches.