This contribution provides an overview of our work on modelling biological processes focussing on the example of programmed cell death, also called apoptosis. Apoptosis is a molecular programme present in all cells of multi-cellular organisms. It is crucial during development and for cell homoeostasis in the adult. Misregulation is implicated in severe diseases. We review an ordinary differential equation model describing core processes of apoptosis signalling and the idea of viewing the life and death decision as a bistable system. Then, we show how small differences in model parameters can give rise to observed population heterogeneities. Employing a new conceptional modelling framework, we further show how a single model can describe a population and how stable steady states then translate into invariant sets employing a local notion of stability. (C) 2008 Elsevier Ltd. All rights reserved.