This chapter discusses the Global Navigation Satellite System (GNSS) error sources in real-time kinematic positioning (RTK). This chapter mainly introduces some errors that affect the RTK accuracy. The GNSS observation equations are given firstly, where the errors are all given. The errors can be mainly divided into three parts: the satellite-related errors (satellite ephemeris error, satellite clock offset, satellite hardware delay, phase windup, phase center offset (PCO), and phase center variation (PCV) of the satellite antenna), the signal propagation errors (ionospheric delay and tropospheric delay) and the receiver-related errors (receiver clock offset, receiver hardware delay, multipath effect and receiver noise, phase windup, PCO, and PCV of the receiver antenna). Due to the characteristics of RTK, orbit error and clock offset, ionospheric delay, and tropospheric delay are discussed in detail. Finally, since other errors can be basically eliminated or ignored, multipath effect cannot be mitigated. This chapter will dedicate relatively substantial coverage to multipath effects.