Chapter 1 represents a sound structure in logical manner to provide wide knowledge of diffusion. Diffusion is useful to engineers, chemists, life scientists and to our daily lives. Chapter 2 contains the discussion about the group theoretic similarity techniques in general. Today there are number of similarity techniques found in literature out of which (1) Free Parameter, (2) Separation of Variables, (3) Dimensional Analysis, (4) Group theoretic are frequently used.Chapter 3 deals with the problems of one-dimensional diffusion equation. The diffusion equation describes the solute transport due to combined effect of diffusion and convection in a medium. It is a partial differential equation of parabolic type, derived on the principle of conservation of mass using Fick's law.