Continuous change changes everything; it introduces various uncertainties, which may harm the sustainability of software systems. We argue that integrating runtime adaptation and evolution is crucial for the sustainability of software systems. Realising this integration calls for a radical change in the way software is developed and operated. Our position is that we need to Design for Sustainability. To that end, we present: (i) the AdEpS model (Adaptation and Evolution processes for Sustainability) to handle and mitigate uncertainties by means of integrating runtime adaptation and evolution, and (ii) a set of engineering principles to design software systems that facilitate the application of the AdEpS model to build sustainable software.