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Physical mechanisms which limit the power handling of YBa2Cu3O7−x films and devices are discussed in terms of a quantitative classification scheme. The possible limitations are devided into magnetic or thermal, and global or local in nature. Analytical estimations are compared with measurements of YBa2Cu3O7−x films (Ø = 1″–2″) using a niobium-shielded sapphire resonator at 19 GHz, and disk resonators at 2 GHz. Magnetic effects are found to play an essential role in nonoptimized films in terms of weak-links, and in high-quality films if the lower critical field Bcl is reached. The majority of films and disk resonators appear to suffer from microwave heating. Global heating appears predominantly at CW operation. Local heating results mainly from defects in films of medium quality. Defect-induced quenches are observed at moderate field levels, sometimes resulting in an irreversible degradation of the power handling.