I discuss the physical mechanism of fast radio bursts based on observational constraints and theoretical modeling. First, within the magnetar model of FRBs, I discuss mounting evidence of the magnetospheric origin of at least some bursts. I argue that FRBs can propagate within a dynamically distorted magnetosphere with relativistic particles. I present a mechanism involving inverse Compton scattering of low-frequency waves to generate coherent emission in FRBs. Finally, I discuss several other possible physical channels to produce some FRBs.