Case 1  Dependent variables are functions of 1 independent variable which takes on only discrete values  Suitable for modeling systems with discrete time characterization  General equation formulation: Z k+1 = g k (Z k, Z k-1,…,Z k-nk ; u k, u k-1,…u k-mk ; θ k ), -∞ < k < ∞, u is a known sequence, θ is parameters