A modular fiber optic cassette system, method and cassette is disclosed. The system comprises a case defining a tray receiving space and an opening to a tray receiving space, at least one tray slideably received within the case for movement between a stored position wherein the tray is inside the case and an accessible position wherein a front end of the tray is in front of the opening and outside of the case, a plurality of fiber optic cassettes arrangeable side by side along the tray front end, a first of the cassettes comprising a first cassette width of one of one, two, three, four or six standard width units and a second of the cassettes comprising a second cassette width different from the first cassette width and one of one, two, three, four or six standard width units, and a fastener assembly for removeably securing each of the cassettes to the tray.