- We review our recent work on the nonclassical radiation field of a one-atom maser. The maser cavity is cooled to 0.5 K to exclude thermal photons and has a quality factor of 3 1010. Velocity-selected Rydberg atoms pump the maser. The field inside the cavity is measured via the fluctuations in the number of atoms leaving the cavity in the lower maser level which are up to 40% below the Poisson level. This corresponds to photon-number fluctuations 70% below the vacuum-state limit. The one-atom maser is the only maser system which leads to nonclassical radiation even when pumping processes with Poissonian statistics are used. We analyse and propose new experiments based on the one-atom maser leading to new studies of the quantum measurement process. 1991 IOP Publishing Ltd.