COMPET is an innovative implementation of a small animal PET scanner using a novel block detector geometry, allowing for a high resolution and high sensitivity. One detector block is built up from layers of long LYSO crystals. Perpendicular and interleaved between the crystals, Wave Length Shifting (WLS) fibers are used. The scintillation light created by a gamma ray interacting with a crystal is measured with Geiger mode Avalanche Photo Diodes (GAPDs) at one end of the crystals. A small part of the scintillation light escapes the crystals and enters the WLS, where it has a certain probability to be absorbed and re-emitted at a longer wavelength. This light is measured at one end of the WLS by a GAPD. With this setup, the point of interaction (POI) of the gamma ray is deduced, allowing for the 3D reconstruction of the interaction point between the gamma ray and the detector. Thus, not only photoelectric interactions are used to reconstruct the line of responses (LOR) for each event, but also Compton scattered gamma-rays are included. Using 4 such modules, the total detector comprises a total amount of 1080 readout channels, where 600 are used for the crystals and 480 for the WLS. The central point source resolution was deduced from Monte Carlo simulation to be below 1 mm FWHM in transaxial direction. The sensitivity to detect coincident gamma rays emitted at the centre of the field of view is up to 16%. With its compact geometry, high point source resolution, high sensitivity and its low amount of readout channels, the COMPET detector geometry provides a promising detector layout for future preclinical PET scanners.