An autoclave comprises an enclosure 10 defining a chamber 12, steam supply means 20, 22 arranged to supply steam to the chamber, steam venting means 34, 40, 58, 60 arranged to vent steam from the chamber, and control means 100, 102 arranged to control operation of the autoclave. The control means comprises a first controller 100 and a second controller 102, each controller being arranged to define a respective set of conditions required for a change of state of the autoclave, to monitor a parameter of the autoclave and determine therefrom whether its respective conditions have been met, and the control means is arranged to allow the change of state only if both sets of conditions have been met.