The last decade has seen tremendous experimental progress in gamma-ray astronomy much beyond simple source detection (e.g., see Rieger et al. 2013, Funk 2015, Madejski and Sikora 2016 for a review). In many cases, detailed spectral and timing characterization have become possible allowing one to probe deeply into the nature and physics of these sources. In particular, gamma-ray astronomy has by now matured and progressed further in the field of time-domain astronomy, utilizing source variability on timescales over many orders of magnitudes, from a decade down to a few minutes and shorter. Instruments such as the Fermi Large Area Telescope (Fermi-LAT), the High-Altitude Water Cherenkov Gamma-Ray Observatory (HAWC) or the First G-APD Cherenkov Telescope (FACT), for example, have opened up the possibility for unbiased long-term (timescales up to several years) studies of bright astrophysical objects in the high energy (HE; > 100 MeV) and the very high energy (VHE; > 100 GeV) domain, respectively (e.g., Thompson 2018, Abeysekara et al. 2017, Temme et al. 2017), while modern Imaging Atmospheric Cherenkov Telescopes (IACTs) have demonstrated their excellent capabilities to characterize VHE flaring states down to below sub-hour timescales.
This paper focuses on some of the key issues and conceptual developments concerning the timing characteristics of AGN (including rapid variability, log-normal flux distributions, power-law noise, and quasi-periodic oscillations) at gamma-ray energies.