An expandable downhole tool such as an under-reamer or a stabiliser includes a mandrel mounted for linear slidable movement relative to a main body. The mandrel, together with one or more travel blocks, cause movement of a working component in an axial direction. One or more outwardly extending guide paths slidably interact with the working component to cause the working component to move between a withdrawn inoperative position and an outwardly deployed operative position. Upon increase of hydraulic pressure from below the tool, the mandrel is urged against a spring biasing the deactivated position to move axially and cause the working component to take its operative position. Upon decrease of hydraulic pressure, the spring moves the mandrel axially to cause the working component to take its withdrawn inoperative position.