I will briefly review formation mechanisms for kilometer-sized solids, i.e., planetesimals, in circumstellar disks of gas and solids.  Under certain conditions, physical collisions between particles result in growth by coagulation.  Alternatively, if small solids settle to a dense midplane layer, planetesimals may arise from self-gravitational collapse.  Turbulence is well-known as a driver of accretion disk flows, but is also important in the "microphysics" of planetesimal formation.  The stirring of particles affects impact velocities, and is feared to inhibit gravitational collapse. I will also present recent work on a novel and robust type of turbulence  which arises spontaneously due to the drag forces between particles and gas. The relevance of overdensities, and other aspects of the turbulent flow, to planetesimal formation will be discussed.