  Precipitation patterns generated by A^{+}+B^{-} --> C type reaction-diffusion
processes are studied. It is shown both theoretically and experimentally that
the patterns can be controlled by an appropriately designed, time-dependent
electric current in the system. We describe examples of current dynamics
yielding periodic bands of prescribed wavelength, as well as more complicated
structures. The pattern control is demonstrated experimentally on the
reaction-diffusion-precipitation process 2AgNO_3+K_2Cr_2O_7 -->
Ag_2Cr_2O_7+2KNO_3 taking place in a gel.
