In this paper, we aim to postulate the conditions of existence of permanent travelling wave solution in reaction diffusion systems subjected to initial, boundary or initial-boundary conditions. In a concomitant way we present for a method that allows us to treat initial, boundary or initial-boundary value problems. It is based on finding approximate analytical solutions starting from the formal exact ones. That is by constructing the Picard iterative sequence of solutions and proving a theorem for,the uniform convergence of this sequence. This sequence is then truncated at first, second or higher approximations. The relative error estimate between approximate analytical solutions and some known exact solutions are of the same order as the error between numerical solutions and the exact ones. We should mention that numerical schemes treat only initial-boundary value problem. It is found that the necessary conditions for the presence of travelling wave in the form of u = u(x − ct) is that the initial (or boundary) conditions at the extreme points of domain of definition of the problem have to be different. It is also shown that the sufficient condition is the presence of an advection term, with coefficient which is a constant or a function in the dependent variable, in the reaction diffusion equation.