Dynamical processes with particular reference to opposite phenomena as growth and decay, may be translated into the mathematical language of difference equations or recursive equations. Such equations can be treated in the discrete case(where the principal mathematical instrument is furnished by geometrical progressions) or in the continuous case (where the mathematical instrument is particularly represented by exponential functions and logarithms). The variety of problems and of resolution methods renders the proposed material apt to be used in a significant way in the high school.