A variable ratio transmission having an infinitely variable ratio, for enabling a tidal turbine (10) or the like running at relatively low but continuously varying speeds, to drive smoothly a constant speed machine, such as a synchronous generator or the like, without excessive transient torques. A preferred transmission includes a primary differential gear train (120), having a driven input (50), and a driving output, wherein the ratio of the input speed to output speed is variable, and wherein said ratio is governed by the reaction torque provided by speed controlling gearing (150) within the primary differential (120), the transmission including a secondary differential gear train (220), which is in rotational communication with the input (50) via a first route (115), and is in rotational communication with the speed controlling gearing (150) via a second different route (125), wherein the secondary differential gear train includes two parallel power paths.