A variable ratio transmission is described having an infinitely variable ratio, for enabling a wind or 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 also 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 P' and P'', one path including a hydraulic bypass having two hydraulic units(180, 500) in a hydraulic circuit, the power transmitted and direction of power transmitted in the hydraulic circuit of the bypass path P' being selectively variable in use to vary the speed in path P'', thereby to alter the power in the second route and to alter the reaction torque of the speed controlling gearing and thereby to govern said ratio. The transmission makes use of the first and second routes for the whole input speed range.