A modular fiber optic cassette and tray assembly is disclosed. The assembly comprises a tray engaging part which engages with a cassette engaging part on the tray and such that the cassettes may be arranged horizontally side by side on the trays. The cassettes are mounted to a front of a tray and such that a portion of each cassette is positioned in front of the tray. In an embodiment, a release mechanism such as a push button is provided on top of each cassette for disengaging the tray engaging part from the cassette engaging part. Still in another embodiment a plurality of trays and cassettes are provided stacked on top of one another.