This textbook provides fundamental theoretical concepts for the understanding, the modelling, and the optimisation of energy conversion and storage devices. The discussion is based on the general footing of efficiency-power relations and energy-power relations (Ragone plots). The book is written for engineers and scientists with a bachelor-degree level of knowledge in physics. It contains: an introductory motivation of the topic a review on equilibrium thermodynamics a primer to linear non-equilibrium thermodynamics and irreversible processes an introduction to endo-reversible thermodynamics the basics on the theory of Ragone plots derivations of efficiency-power relations or Ragone plots for illustrative examples like heat engines, batteries, capacitors, kinetic energy storage devices, solar power, photodiodes, electro-motors, transformers, and flow turbines an excursion to impedance matching and the optimization of technical devices with respect to economic and related objectives　 All concepts and models are reduced to their basics in order to allow non-experts to easily grasp the essentials. Thomas Christen (PhD ‘94 in theoretical physics) works as a senior principal scientist in Corporate Research of ABB Ltd, Switzerland. He periodically lectures on physics in industry at the University of Basel.