The basic theory of turbulence is reviewed, especially as it applies to the smallest scales, in the parameter range typical of the ocean above the thermocline but away from the direct effect of surface waves. The main time and length scales are defined, and related to the environment experienced by particles in the size range of plankton.Two problems are analysed in some detail to illustrate the application of the theory: the differential concentration of almost neutrally buoyant particles, and the size of the sensory and feeding horizon of current feeders.