This chapter explores the application of channel state information (CSI) for indoor Wi-Fi based localization, highlighting advantages of CSI over the received signal strength indicator (RSSI) methods due to its fine-grained subcarrier-level information. First, the definition of CSI and the operation of obtaining CSI from hardware are described in detail. Second, since carrier frequency offset (CFO), sampling frequency offset (SFO), and packet detection delay (PDD) cause phase errors in CSI measurements, phase compensation methods and coherent smoothing methods are provided. Third, interference reconstruction and cancellation methods are described to obtain subtle target reflection parameters overwhelmed by strong propagation paths in complex scenarios such as through-the-wall. At last, Wi-Fi indoor localization can be achieved by utilizing estimated parameters such as angle of arrival (AoA), time of flight (ToF), and path length change rate (PLCR) combined with statistical and geometric analysis.