ABSTRACT Cyber-Physical Systems (CPS) is a combination of computation and networking systenis with the physical systems that interact together in complex ways and rising uncertainty. Common examples of CPS include power generation grid, chemical industries, water distribution sector, and petroleum industries. Many governments, companies and research institutions nowadays are seeking competitiveness over this technology. Security is one of the key concerns in CPS due to their cyber physical nature and interaction. Consequently, any successful attack to CPS will result in unimaginable consequences which would make cyber security risk assessinent in CPS a challenging task. To safeguard the CPS resources and cover the growing security issues, a well-designed risk assessment approach is required. The available risk assessment approaches in the area of cyber security may not be applied directly to the CPS since they are different in many aspects. Recently, some approaches have been developed for assessing the CPS risk, and because of the critical role of Cyber-Physical Systems, the development of risk assessment approaches is of utmost importance. This thesis will contribute in the following ways: Firstly, it will explore, review and analyse some current risk assessment approaches for the CPS and Smart Grid as one of the CPS domains. Secondly, a comprehensive list from extensive literature review of the available risk assessment approaches or methodologies will be presented for the CPS and Smart Grid. Thirdly, a risk assessment approach will be developed based on the findings. The application of the proposed approach will be demonstrated on Smart Grid environment as an example and one of the CPS' most critical domains.