Swimming in Complex Fluids

We review the literature on swimming in complex fluids. A classification is proposed by comparing length and time scales of a swimmer with those nearby obstacles, interpreted broadly, extending from rigid or soft confining boundaries to molecules which confer bulk fluid stresses. third dimension concentration swimmers, incorporates fluids whose complexity arises purely collective motion organisms. For each eight system classes we identify provide background describe modern research findings. While some have seen great deal attention for decades, others remain uncharted waters still open awaiting exploration.