This paper deals with fundamental change processes in socio-technical systems. It offers a typology of changes based on a multi-level perspective of innovation. Three types of change processes are identified: reproduction, transformation and transition. 'Reproduction' refers to incremental change along existing trajectories. 'Transformation' refers to a change in the direction of trajectories, related to a change in rules that guide innovative action. 'Transition' refers to a discontinuous shift to a new trajectory and system. Using the multi-level perspective, the underlying mechanisms of these change processes are identified. The transformation and transition processes are empirically illustrated by two contrasting case studies: the hygienic transition from cesspools to integrated sewer systems (1870-1930) and the transformation in waste management (1960-2000) in the Netherlands. © 2007 Elsevier Ltd. All rights reserved.