Abstract: A derrick (1) for drilling and maintenance of oil and/or gas wells comprises a portal-shaped derrick structure installed on the drilling deck, consisting of an upper traverse (2) and two spaced-apart, substantially parallel, upright derrick components (3,4) which together define a free space (5), within which a hydraulic elevator (6) connected to a drilling machine (18) for the drill string (15) is adapted to travel along stationary, upright rack rails (10). The vertically movable elevator (6), in a manner known per se, is driven by means of hydraulic motors having gears (9) that engage with the rack rails (10).