The software QBlade is developed as an open source framework for the simulation and design of wind turbines. QBlade utilizes the Blade Element Momentum (BEM) method for the simulation of horizontal axis-and a Double Multiple Streamtube (DMS) algorithm for the simulation of vertical axis wind turbine performance. For the design of custom airfoils and the computation of airfoil lift-and drag coefficient polars the viscous-inviscid coupled panel method code XFOIL is integrated within the graphical user interface of QBlade. Additionally, a module for the extrapolation of airfoil polars, beyond the stall point, for a 360 range of angles of attack is integrated. The resulting functionality allows the use of QBlade as a comprehensive tool for wind turbine design. QBlade is constantly being maintained, validated and advanced with new functionality. This paper describes the software and its modules, at the current state, in theory and application.