First, we consider non-linear discrete-time and continuous-time systems with unknown inputs. The problem of reconstructing an input using the information given by an output equation is investigated. Then we examine a control problem for non-linear discrete-time hereditary systems, i.e. the problem of finding a control which drives the state of the system from its initial value to a given desired final state. The methods used to solve these problems are based on the state-space technique and fixed-point theorems. To illustrate the outlined ideas, various numerical simulation results are presented.