The deformation of materials over time at variable stresses and temperatures is accompanied by the occurence of a number of complex nonlinear mechanical and thermal phenomena which, as a whole, are difficult to describe by means of the integral and integrodifferential operators normally used in creep theory. Here, we introduce operators of a special structure and use them to obtain governing creep relations that reflect most of the phenomena seen experimentally in both the linear and nonlinear strain regions. A comparison is made with experimental results and the relations are used to solve a practical problem.