Integrated photonics is in high demand for various applications, such as optical communication, microwave photonics, optical sensing, and quantum information processing. Different material platforms have been explored for implementing photonic integrated circuits (PICs), such as silicon, indium phosphide, lithium niobate (LN), and more recently thin-film lithium niobate (TFLN). Choosing a platform depends on the specific requirements of the application. TFLN offers numerous advantages, including very strong electro-optical and nonlinear optical effects, and a broad spectral range without significant material loss. Particularly, TFLN is suitable for high-speed and low-power electro-optical modulation. LN is a widely used electro-optic material in telecommunication and TFLN has shown considerable potential for improved PICs, some of which are presented in this work. First, the feasibility of ultrahigh-speed electro-optic modulators using TFLNPICs is shown. Design guidelines are provided for optimizing key performance metrics, resulting in devices with a 3-dB modulation bandwidth of up to 400 GHz. Next, TFLN electro-optic modulators with an extrapolated 3-dB bandwidth of 170 GHz and low half-wave voltage-length product of 3.3 V.cm are experimentally demonstrated. The trade-off between bandwidth and voltage is addressed by the non-symmetric positioning of waveguides with respect to the radio-frequency (RF)electrodes, and by incorporating a dielectric buffer layer. Furthermore, integrated microwave-to-optical converters (MOCs) are demonstrated on TFLN for microwave photonics applications. The proposed design incorporates two types of antennas — patch and bow-tie — that can operate in two different frequency bands: K-band and W-band. The demonstrated TFLN MOCs are used in an optical system for photonic down-converter applications. These systems convert a high-frequencywireless RF input into a low-frequency signal (e.g., 100 MHz). This technique facilitates easier analysis and detection in the field of electromagnetic wave measurement.