A rotor lock system (20) for securing a main shaft assembly of a wind turbine (2) in a substantially stationary position, the main shaft assembly comprising a main rotor shaft (16) supported by a base frame (14), and the rotor lock system comprising: a locking disk (22), associated with the main rotor shaft (16), and provided with a plurality of locking apertures (26); and a locking unit (24) comprising a first end (28) arranged to engage with the locking disk (22), and a second end supported by a mounting feature (38) associated with the base frame (14), wherein the locking unit (24) is configured to be adjustable relative to the mounting feature (38) so that the first end (28) moves linearly with respect to the mounting feature.