Single photon detectors for biology: present and future

A summary of the status of present technology for the detection of single photons is presented with a view towards applications in biophotonics. Included are careful discussions of the numerous problems that can be encountered and how to get around them with the hope that this will be of help to biologists interested in doing work in the field of biophotonics. Emphasis is placed on traditional devices, but the field is one which is continuously developing and we review the status of new and very interesting technologies which are becoming available. The paper is meant to be fairly self-contained and assumes no extensive knowledge of the physics of photodetection.