This lecture introduces energy system modelling, focusing on the design and operation of energy systems, energy needs, required infrastructure, costs, and emissions. It covers the use of computer models for optimization, involving researchers, engineers, governments, and companies. The lecture discusses scenarios, predictions, and the computational complexity of energy system models, emphasizing the challenges of capturing spatial and temporal details. It also addresses the complexities, variations in prices, socio-economic effects, and the impact of assumptions on model accuracy and controversies.