Results from different methods searching for Quasi periodic oscillations (QPOs) in blazars will be shown, indicating no significant evidence for periodic signals beyond the noise level. Blazars are a subclass of active galactic nuclei (AGN), and are highly variable objects. QPOs, which might originate from a binary black hole located at the AGN core, have been found in some blazar light curves. For the blazars Mrk 421 and Mrk 501, we test the possible QPO behaviour using a variety of methods (generalized Lomb-Scargle Periodogram, CARMA, Wavepal and A-T plane), studying in detail systematic effects. We use gamma-ray light curves from FACT, a ground-based imaging air Cherenkov telescope and Fermi-LAT, a gamma-ray satellite. Furthermore, we explore the possibility to search for periodicities with the information field theory.