Changes in European Energy Policy in form of liberalisation, sustainability and security of supply measures have profoundly altered the European gas market structure and hence the organisational environment and requirements of incumbents active within. This thesis analyses how these companies reacted to such changes and strategically behaved over the course of time in order to survive and be successful in their new environment. Based on the categorisation of environmental influences and organisational goals into market and non-market ones and in order to capture the complexity of the empirical research setting, a theoretical framework consisting of a business- and an institutional-based approach is established, complemented by a resource-based perspective to allow comprehensive analysis. This again serves as a basis for the development of a model that enables the identification, examination and explanation of organisational development paths over time which here are determined through case study research. Apart from revealing different paths of organisational behaviour this thesis contributes to the integration of institutional theory and economic-/ business-based perspectives.