Engineers often make changes during system development in order to correct design weaknesses. If done well this results in reliability growth as development continues. Since no reliability-growth model is adequate for all time (any growth model is valid only for some finite amount of time) it is prudent to use a different reliability model between each two design modifications. This paper assumes that the reliability growth model between each two design modifications is either a Poisson process or power-law process. Maximum likelihood estimates of the parameters are obtained for two cases: (1) no constraints on parameters; and (2) some constraints on parameters.