Confinement and manipulation of photons using microcavities have triggered intense research interest in both basic and applied physics for more than a decade. Prominent examples are whispering gallery microcavities which confine photons by means of continuous total internal reflection along a curved and smooth surface. The long photon lifetime, strong field confinement, and in-plane emission characteristics make them promising candidates for enhancing light-matter interactions on a chip. In this book, we will introduce different ultra-high-Q whispering gallery microcavities, and focus on their applications in enhancing light-matter interaction, such as ultralow-threshold microlasing, highly sensitive optical biosensing, nonlinear optics, cavity quantum electrodynamics and cavity optomechanics.