Turbines are a major working component in a gas turbine, which operate in a hostile environment in terms of the extreme temperature and unsteady flow and often need cooling air to provide structural integrity and guarantee component life. The different turbine roles for propulsion and power generation are explained. In this chapter, we describe the flow process through a turbine stage and derive the simple (uncooled) efficiency parameter. We show how to construct turbine velocity diagrams and discuss their use to determine responses to change. We derive and explain the use of the non-dimensional turbine stage characteristics: stage loading coefficient, flow coefficient and reaction. The Zweifel coefficient is also used to quantify blade loading. We introduce the Smith Chart as the first performance correlation of efficiency versus stage loading and flow coefficients. The development and structure of mean line design codes is laid out and an account of flow features in a blade passage and their associated loss elements is presented. We then work through an example of a turbine design envelope integrating multiple constraints to derive a solution space. Lastly, we present the vaneless counter-rotation turbine as an alternative design concept.