Over the past few years, much work has been devoted to the study of microcavities in photonic crystals (PCs). High-quality factor (Q) PC microcavities exhibit attractive properties since they can confine light in wavelength-scale dimensions, making them potentially useful not only for photonic integration but also in quantum optics. Most of the high-Q PC microcavities are realized in slabs (PCSs), which are two-dimensional (2D) PCs with a high refractive index core layer providing light confinement in the third dimension. The key to design high-Q cavities is to reduce the radiation losses, i.e., to minimize the spatial Fourier components above the light line of the PCS. This chapter will focus on high-Q PC microcavity, including design rules, characterization methods, device demonstrations, and applications.