The basic objective is the presentation of a description of rate‐sensitive plastic materials which agrees with events on the microscopic level and with macroscopic experiments and which is internally consistent. We utilize the framework of the thermodynamics with internal state variables formulated recently by Coleman and Gurtin. The internal state variables in the present theory are considered to be average quantities related to properties of the crystal defects in the material. Rate‐dependent phenomena, (e.g., a time‐dependent stress‐strain relation, creep, etc.) are studied in detail. The theory is illustrated with a simple example.