Abstract: Blood flow monitor apparatus includes a transducer having a skin-engaging face from which first and second light guides respectively lead to a photodetector and light source, the second light guide forming an array surrounding the first at said face. There can be a plurality of second light guides leading to respective individual sources or, by bundling guides of fibre optic form, to a single source. In the latter case the source can be remotely connected by a fibre optic cable to a separate skin probe. Infra-red operation is preferred. The apparatus will also normally involve a visual display of, and an alarm responsive to attainment of a preset level for, at least one characteristic of the detector output. The first time differential is a characteristic of particular interest, and a presently preferred characteristic is the ratio between the maximum and minimum values of this differential.