The transition to a clean energy future requires thorough understanding of increasingly complex interactions between conventional generation, network equipment, variable renewable generation technologies (centralised and distributed), and demand response. Secure and reliable operation under such complex interactions requires the use of more advanced power system modelling and simulation tools and techniques. Conventional tools and techniques are reaching their limits to support such paradigm shifts. This paper provides an overview of commonly used and emerging power system simulation tools and techniques. Applications of these tools ranging from real-time power system operation to long-term planning are also discussed. Various approaches to gain confidence in the accuracy and applicability of the simulation models are presented. The paper then discusses emerging trends in simulation tools and techniques primarily stemming from the transition to a power system with increased penetration of inverter-based resources as these are used in variable renewable energy technologies.