It has been over two decades since 3D photonic band gaps were proposed to strongly confine light in a small space , but the proposed strong light confinement was achieved just recently in a slightly different form. Recent progress in nano-fabrication technologies enabled high-quality 2D photonic crystals, and various high-Q microcavities have been now realized in them. Among these photonic crystals cavities, only a certain type of cavities experimentally exhibit Q higher than 1 million, which can be categorized as a modulated mode-gap cavity. This type of cavities have several remarkable features besides their high Q, and they have been adopted to various applications and devices. This talk will present these features and possibilities of this particular type of cavities .