We propose in the paper an analysis of testing and the taking into account of testing in the assembly systems design. We define three kinds of variables: the variables of aiming kind, the variables of operative kind and the variables of acting kind. The method to design a testing system that we proposed is the process of determination of these variables. Starting from some variables of aiming kind, representing what we want to test on the product in term of functionalities, we determine the variables of operative kind. The testing system design is ended when the choice of the variables of acting kind, performed from each variable of operative kind previously determined, is made. The problem of the choice of these variables is explained.