Electricity supply industries (ESI) around the world are subject to structural and regulatory change. The environmental implications of these changes will depend, largely, on future investment within cleaner technologies. As developing countries (Des) increase levels of electricity supply, the incentives for investment in clean technologies is particularly important. Policy-makers wishing to promote renewable electricity technologies (RETs) in Des need to understand the nature of technological change in large technical systems (LTS). Broadly this thesis adopts the view that technological change is the outcome of the complex interaction of technical, economic and political factors. Initially technological change in LTS is shaped by social and political factors. As the system increases in both size and complexity driven by economies of scale and scope, and through co-evolution of technical and institutional features, it exhibits 'momentum,' whereupon technological change tends to be 'incremental' and autonomous. Through problem-solving activities to address 'reverse salients' the system evolves on a 'technological trajectory,' its path confined by technological and economic boundaries defined by the prevailing 'techno-economic' paradigm. Subsequently new technologies such as RETs, with characteristics different to those of the 'Traditional Electricity System Trajectory' (TEST) are unlikely to be favoured. Restructuring the electricity system provides a discontinuity in its momentum, allowing the drivers and interactions of different stakeholders to be more transparent. During such periods of instability there are important opportunities for systemic change through meaningful policy input. The socio-economic importance of electricity supply in Des further increases the 'technoinstitutional complexity' within the electricity system, and so resistance to restructuring. The thesis argues that restructuring of the ESI is a necessary, but not sufficient requirement for commercialisation of RETs. Rather policies supported by legislation should ensure that conservative techno-institutional mechanisms are replaced by ones that encourage a 'Balanced Electricity System Trajectory.' The BEST framework incorporates 'economies of the system' as a driver and is characterised by distributed technologies including small-scale and modular generation and sophisticated control technologies. As well as being characterised by flexible control in the technical sense, the BEST model is also characterised by flexible institutional arrangements.