Looking at cities these days, we could observe the advanced level of developments and integration of technologies in all sectors and systems. However, at the same time, we are living in a changing world. In recent years, humans started to realize the cost of this development and the burden laid on the planet due to human activities. This increase in complexity, challenges, and uncertainties fueled the need to adopt a resiliency-based design approach. In this book chapter, we address the resiliency in energy systems definition, some assessment methods, and the integration of renewable energy sources on it. Also, we added two real case studies that further explain resiliency repercussions and enhancement practices. Nevertheless, considering the importance of mutual dependency and security of critical systems, we highlight these nexuses between energy and some of the critical systems within the scope of resiliency.