Since deformation of crystalline rocks is governed by laminar flow, stream functions may be applied to study the development of ductile deformation patterns. The chapter is started with the phyiscal meaning of streamlines, flownets an streamtubes. After that, the stream function can be derived from the velocity field of any particular flow. After some computations, a more general stream function is defined and used to model finite deformation patterns. Finally, the components of teh velocity gradient tensor are outlined in section 13-6.