Abstract: Particles in a fluid are counted by passing the fluid through an optical cell and an image of said particles is projected onto an array of charge coupled devices such that the area of the image of each particle at the said array is approximately the same as the area of at least a single charge coupled device, and signals from the individual charge coupled devices are processed to provide information concerning at least the number of said particles passing through the optical cell. A particle counter disclosed comprising an optical cell (1) through which is passed a fluid containing particles to be counted, means (2) for illuminating the particles in the optical cell and optical means (3) for providing an image of the particles on an array of charge coupled devices (5) such that the area of the image of each particle at the said array is approximately the same as the area of at least a single charge coupled device. The particles may fluoresce and a transmittance filter (4) may be provided.