The present chapter is concerned with those Dynamical Systems where the time variable is considered as a discrete variable, i.e. the variable or the vector of variables of the model is recorded at fixed time intervals (for example, each year, each month, each week, etc.). This gives rise to the so-called (finite) difference equations and to systems of difference equations. In general, difference equations and systems of difference equations are more difficult to treat, from an analytical point of view, than differential equations. Hence, also in the present chapter, only the basic notions concerning the existence of solutions and the stability of equilibrium solutions are given. In particular, the last section (i.e. Sect. 7.6) contains some elementary economic applications of difference equations.