I present a theory of time-dependent plastic deformation of solids. I deriv
e a general equation of motion for the stress probability distribution func
tion P. I consider in detail uniaxial tension and show how this case can be
(approximately) mapped onto a simple one-dimensional spring-block model. I
study stationary creep and the linear and nonlinear response of a solid bl
ock to an imposed oscillatory strain. The theory is in good agreement with
experimental data.