Spread spectrum (SS) communication systems have been studied and developed for commercial applications such as mobile communications, consumer communications and so on. The reason is that SS communication systems have various characteristic features, such as, robust immunity to interference and jamming, achieving privacy in communication and random access capability (spread spectrum multiple access: SSMA). The performance of the systems, mainly, depends on modulation method of spreading the spectrum. This paper introduces recent several methods of modulation in SS communication systems. First, direct-sequence (DS), which is a fundamental modulation in SS communication system, and M-ary/SSMA, which can increase the number of multiple access users, are described. For increasing the data rate per user, parallel SS, parallel combinatory SS and a code division multiplex using orthogonal Manchester coded M-sequences are introduced. Second, frequency hopping (FH), in which the spectrum of signal is spread by hopping carrier frequency, and related systems are shown. Finally, from a viewpoint of communication theory, spectral efficiency (i.e., data rate the number of multiple access users/bandwidth) of systems relating spreading codes are discussed. As a result, maximum spectral efficiency can be obtained by appropriate spreading factor for each spreading code.