Crystalline lithium niobate (LN) is considered the "silicon of photonics" because of its outstanding optical properties, including a broad transparency window and high piezoelectric, acousto-optic, second-order nonlinear, and electro-optic coefficients, which are critical for photonic integrated circuit (PIC) applications. Recent breakthroughs in nanofabrication technology facilitate a variety of high-performance integrated photonic devices on thin-film LN, such as ultrafast electro-optic modulators, broadband optical frequency combs, and high-efficiency optical frequency convertors.