Neutrino detectors in water and ice can be described as a grid of optical sensors, called optical modules, which are sensitive to the Cherenkov radiation emitted by charged particles produced by neutrino interactions. The paper deals with a general overview on the different designs of the optical modules of the current neutrino detectors, with a particular focus on the design and main advantages of the multi-PMT solution, selected for the optical module mass production in the frame of the IDMAR project.