Particle collisions and filter cake collapse are observed through a microscope during the formation of filter cakes. Cake filtration is widely applied throughout industry for fluid-particle separations. Models and theories have been developed to describe and predict filter cake performance, but there is little literature on direct observation of particle packing and cake formation. A filter assembly the size of a microscope slide is placed on the microscope such that the slurry flow and cake formation is across the field of view. The cake formation is recorded on video tape. Photographs of the video image show particles in the slurry colliding with and sometimes adhering to other particles when forming the filter cake. The photographs also show the cake structure collapse as the cake compacts or compresses.