The Square Kilometre Array (SKA) is an ambitious project to build a radio telescope that will enable breakthrough science and discoveries not possible with current facilities. Initially, two telescopes are planned, SKA1-mid and SKA1-low, built over two sites in southern Africa and Western Australia, respectively. Constructed in two phases: SKA1 is being designed now; SKA2 is planned to follow. This paper describes the science capabilities and overall design of SKAl-mid — given the present stage, a transition from preliminary design to detailed design.