An overview is provided of recent theoretical developments that generalize stable population theory to open population with variable fertility and mortality rates. Applications in mortality and fertility estimation useful for developing countries are then deduced from the generalized stable equations, sometimes called r-variable methods. The general relations extend Lotka's original stable population formulations, but their practical advantages are not always evident. Most of the relations are essentially tautological and do not necessarily provide better results than direct calculation. Most independence between risks. For example, a variable representing the probability of survival excludes the risk of migration, but the assumption of independence is not always realistic. The general relations have permitted introduction of new techniques of evaluation and estimation of demographic characteristics requiring fewer assumptions than the classic methods based on the theories of stability or quasi-stability. But the techniques require intercensal estimation of growth rates by age, which in developing countries are often biased due to defective data. As with all indirect techniques, these methods offer an uncertain alternative to correct and complete collection of data.