This book 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). Efficiency and Power in Energy Conversion and Storage: Basic Physical Concepts, 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