Energy and the environment are major issues of discussion in the last years. Security of energy supply in the future is questionable, as energy reserves are depleting. Moreover, damage has been caused to the environment by human actions. Therefore, sustainability has become a major challenge. Scientists and researchers around the world are dedicated to research towards a more sustainable future. However, thinking and working in parts is not always the best way to tackle problems. Integration is a promising idea, which can assist towards accomplishing energy and sustainability goals. Integration is oriented towards systems thinking and can be defined as the combination of energy sources, energy production methods and technologies as well as of the research and policy sectors, aiming at sustainable development. There have been distinguished 6 forms of integration in (Hemmes, 2009) and, based on the work on this thesis, 9 categories of integration have been defined in total: integration of components into a system, integration of energy sources into multi-source multi-product energy systems, integration of industries into ecoparks (Industrial Ecology), integration of new technology into existing technology, integration of sectors, integration of functions, integration of sites, integration of renewable energy into products, integration of sustainability into energy systems design. Multiple examples are given for each category of integration. Literature research as well as a free brainstorming session were used in order to retrieve as many examples as possible. The categorization of integration options was examined from three different points of view: definitions, scale and ownership. The various effects that integration might have are a matter of discussion as well. As it was pointed out, integration can lead to innovative ideas and creative solutions, the generation of niche markets, the creation of new regimes, the introduction of new technology, without renouncing the old one, and the decreased dependence on conventional forms of energy. However, acceptance of new products and regimes is questionable, and the cooperation and responsibility sharing between various parties is also important to consider. Moreover, it is possible that integration will lead to more complex systems and higher product costs or even create ‘lock-ins’ for the further penetration of renewable energy. Nonetheless, in order to enhance the positive results of integration, the way that research is organized needs to change. It is proposed that international cooperation is increased and multidisciplinary teams are created, in order for more communication between researchers to take place. Funding is considered to be crucial. Additionally, the connection between research and applications and the provision of motivation to the private sector are essential. Focus is given on one integration category, namely the multi-source multi-product energy systems. Multi-source multi-product systems (MSMP) are energy systems with multiple inputs and multiple outputs. Inputs in MSMP systems can be renewable or conventional forms of energy. Outputs may also vary: electricity, hydrogen, heating power, cooling power, etc. MSMP energy systems have a lot of advantages, among which the increase of energy efficiency, the reduction of CO2 emissions, the facilitation of renewable energy penetration, the reliability of energy supply, and the flexible utilization of the energy carriers. A category of MSMP energy systems are the hybrid energy systems, which have a combination of renewable and non-renewable energy carriers as input. The interface between the inputs and the outputs of a MSMP or hybrid energy system is the energy hub, where direct transmission, conversion or storage of energy takes place. MSMP energy systems can be studied in two ways: the first option is to choose a flowsheet software and the second option is to construct the conversion matrix of the system. The conversion matrix is a matrix which shows how the inputs of the system are transformed into its outputs. Two different ways of constructing the conversion matrix of the system will be presented and evaluated. It was proved that the approach presented in (Geidl, 2007) was far broader and easy to use, and was thus preferred by the majority of scientists in the field. In order to get more insight on integration and MSMP energy systems, a hybrid energy system is studied. The energy system under study derives from the integration of solar thermal energy and a conventional combined cycle power plant (Integrated Solar Combined Cycle or ISCC). In such a power plant, the solar heat is used at the steam turbine cycle and not at the higher combined cycle efficiency. In this thesis, the ISSC power plant is studied by applying the superheating principle and by using a software developed by TU Delft, CycleTempo. The superheating principle is actually the heating of the steam into a higher temperature, before it enters the steam turbine. The results of the model show that superheating of the steam, at the point of the power cycle where the steam exits the heat recovery system, has a positive effect on the system’s efficiency. However solar tower or dish solar thermal technology should be chosen for the solar field in this case, since they can reach higher temperatures than the parabolic mirrors. ISCC power plants around the world are presented as well. Some of these plants are newly constructed, whereas in other cases the combined cycle power plant existed in advance. Moreover, in the ISCC projects so far, a lot of different parties have cooperated, sometimes not without problems. Therefore, a stakeholder analysis is carried out, which shows the relations between the actors and stakeholders, their interests and the critical points which need special attention during the integration procedure. It is observed that in all cases a similar approach is followed and the consistency of the stakeholder group does not change much. Additionally, interviews with people that participated in integration processes were carried out, in order to gain additional information. The interviews validated the stakeholder analysis results and gave insight to the integration procedure. The main policy advices that emerged from the stakeholder analysis are that, first of all, the problem owner should have a clear vision of what they want to achieve and, secondly, that the thorough knowledge of all the aspects of the situation at hand and a diligent risk analysis can prevent future problems. Integration offers various options for the utilization of renewable energy sources. Our hypothesis is that integration can accelerate the penetration of renewable energy sources and lead the way of transition to energy production based totally on renewable energy sources. This argument is elaborated on in the final chapter of the thesis. It is found out that although integration is a valuable tool in the penetration of renewable energy, it should definitely be supported with additional policies.