Stochastic thermodynamics extends classical thermodynamic concepts like work, heat, and entropy production to individual trajectories of non-equilibrium ensembles. It applies to systems coupled to one or multiple heat baths at constant temperature, such as colloidal particles in time-dependent laser traps, polymers in external flows, molecular motors, and small biochemical networks. For such systems, a first-law-like energy balance can be identified along fluctuating trajectories. Thermodynamic consistency imposes a local detailed balance constraint on noise and rates. Various integral and differential fluctuation theorems, derived from a unified master theorem, constrain the probability distributions for work, heat, and entropy production. These theorems hold particularly strong results for non-equilibrium steady states, including a generalized fluctuation-dissipation theorem involving entropy production. The review covers optimal driving, measurement-based feedback processes, and the relation between dissipation and irreversibility. Efficiency and efficiency at maximum power are discussed for isothermal molecular machines and heat engines using a common framework based on cycle decomposition of entropy production.Stochastic thermodynamics extends classical thermodynamic concepts like work, heat, and entropy production to individual trajectories of non-equilibrium ensembles. It applies to systems coupled to one or multiple heat baths at constant temperature, such as colloidal particles in time-dependent laser traps, polymers in external flows, molecular motors, and small biochemical networks. For such systems, a first-law-like energy balance can be identified along fluctuating trajectories. Thermodynamic consistency imposes a local detailed balance constraint on noise and rates. Various integral and differential fluctuation theorems, derived from a unified master theorem, constrain the probability distributions for work, heat, and entropy production. These theorems hold particularly strong results for non-equilibrium steady states, including a generalized fluctuation-dissipation theorem involving entropy production. The review covers optimal driving, measurement-based feedback processes, and the relation between dissipation and irreversibility. Efficiency and efficiency at maximum power are discussed for isothermal molecular machines and heat engines using a common framework based on cycle decomposition of entropy production.