The detection of ultra-high energy neutrinos and cosmic and gamma rays is used to study some of the most exciting phenomena in nature. The merging of two black holes, the active hearts of distant galaxies, and the mysterious events that produce gamma ray bursts are just a few examples. The flux of cosmic-ray particles at high energies, above 1015 eV, is too low for direct measurements, but instead can be reconstructed from air showers, induced in the Earth’s atmosphere, measured with extended devices on ground. As cosmic-ray observables like the flux or mass composition are always interpreted as a function of energy, a precise and accurate energy measurement is of importance to all cosmic-ray detectors. There are different methods for detecting air showers, of which most can be classified in particle detector arrays and optical techniques. Particle detector arrays measuring the secondary particles at the observation level can be operated around-the-clock, and thus offer the highest exposure and best event statistics. But they are limited by systematic uncertainties from air-shower simulations based on hadronic interaction models beyond the energy range probed by accelerators, which a required for proper interpretation of the data. Especially the muonic component of air showers seems to be poorly described by contemporary models, possibly also distorting the energy scale of the detectors. Optical techniques, detecting the air-Cherenkov or fluorescence light of the electromagnetic air-shower component suffer less from systematic uncertainties of air-shower simulations, but can only operate during clear and dark nights, reducing the statistics by an order of magnitude. To overcome these problems, contemporary observatories combine advantages from the different observation techniques in hybrid detectors. Such type of detectors are presented in this Section.