The review deals with the present-day knowledge of the fundamentals of impurity, transport in tokamaks and stellarators. Emphasis is put on the processes in the edge region, which are of crucial importance for wall-produced impurities. For the anomalous transport model, closed analytic expressions for the stationary case are derived which allow the importance of various transport and plasma parameters to be estimated. Some general features of non-stationary problems are also discussed. In particular, the definitions and essentials of various characteristic times are dealt with. Moreover, questions of diffusive penetration and impurity screening are clarified.