Masing at room temperature is achieved by an apparatus and method that utilize a microwave cavity which exhibits a resonance of sufficiently high Q-factor for maser oscillation, and a resonator structure comprising a masing medium located within a resonant element. The masing medium comprises spin-defect centres. The resonator structure is disposed within the microwave cavity. A magnetic field is applied across the masing medium. An input of microwave radiation to be amplified is coupled to the resonator structure. An optical pump pumps the masing medium, thereby causing stimulated emission of microwave photons. The microwave cavity has an effective magnetic mode volume matching the volume of the masing medium.