A quasi-evolutionary model of socio-technical transitions is described in which regimes face selection pressures continuously. Differentiated transition contexts determine the form and direction of regime change in response to these pressures. The articulation of pressures, and the degree to which responses are coordinated and based on resources available within the regime, define the transition context. Four alternative contexts are described: endogenous renewal; re-orientation of trajectories; emergent transformation and purposive transitions. Agency and power in the governance of regime transformation are analysed. Power to affect change depends on regime membership, the distribution of resources for change and expectations.