Apparatus for monitoring the cure of a bone cement material which is provided in a container. The apparatus includes a radiation source from which radiation is directed towards cement in the container and a sensor for detecting radiation from the radiation source which has passed through cement in the container, and for generating a signal according to the intensity of the detected radiation. A data processor is used in monitoring changes in the intensity of radiation that is detected by the sensor due to changes in the opacity of cement in the container to radiation from the source as the cement cures.