In this chapter, we present the principle of spread-spectrum transmission and demonstrate the basic underlying mechanisms for its desirable features. We first discuss the most popular spectrum spreading method, direct sequence spread spectrum (DSSS). Noting that the desirable features of spread spectrum system rely heavily on the spreading signal design, we then discuss the design of spreading codes. RAKE receiver and multiple user transmission based on DSSS are considered afterwards. We conclude the chapter with a brief discussion on frequency-hopping spread spectrum (FHSS).