The invention relates to a wind turbine (2) comprising: a nacelle (8) provided on the top of a tower (12), a rotor including a hub (6) and a number of blades (4), a main shaft (16) configured to be driven by the rotor about a main axis and supported on the nacelle (8), a generator (28) having a generator rotor and generator stator, and a gear system (25) arranged to increase the rotational speed between said rotor and said generator rotor. The gear system (25) comprises: a fixed ring gear (50), an input member (53) coupled to or driven by the main shaft (16) having a plurality of radially movable tooth segments (52,63) carried in guiding slots and engageable at outer ends with the ring gear (50), a central output member (55) within the input member (53) having an outer eccentric profile acted on and driven by inner ends of radially movable tooth segments (52,63), whereby rotary movement of the input member (53) drives the radially movable tooth segments (52,63) through engagement with the ring gear (50) and effects rotation of the central output member (55).