  This review examines recent theoretical developments in our understanding of
turbulence in cold, non-magnetically active, planetesimal forming regions of
protoplanetary disks which we refer to throughout as "Ohmic zones". We give a
brief background introduction to the subject of disk turbulence followed by a
terse pedagogical review of the phenomenology of hydrodynamic turbulence. The
equations governing the dynamics of cold astrophysical disks are given and
basic flow states are described. We discuss the Solberg-Hoiland conditions
required for stability, and the three recently identified turbulence generating
mechanisms possibly active in protoplanetary disk Ohmic zones, namely, (i) the
Vertical Shear Instability, (ii) The Convective Overstability and (iii) the
Zombie Vortex Instability. We summarize the properties of these processes,
identify their limitations and discuss where and under what conditions these
processes are active in protoplanetary disk Ohmic zones.
