In this paper, we review the history of 2-D photonic crystal slabs with a focus on point-defect cavities that can simultaneously realize ultrasmall modal volumes and ultrahigh quality factors. An important design concept to suppress radiation loss is introduced: the envelope of the cavity mode field should have no abrupt changes and should ideally follow a Gaussian function. Cubic-wavelength-order modal volumes, with experimental Q factors exceeding 11000000, are obtained by tailoring the envelope functions using heterostructure nanocavities. We also discuss light/photon manipulation techniques, which are enabled by the emergence of high-Q photonic crystal nanocavities, including ultracompact channel add/drop filters, ultralow-threshold silicon nanocavity Raman lasers, and strong coupling between distant nanocavities via the adiabatic transfer of photons.