This chapter provides an overview of 2G/2.5G and 3G wireless communication systems, with a focus on security-related aspects. Since the introduction of the first-generation cellular networks in the 1980s, wireless communications have experienced significant growth. The launch of the first commercial GSM network in 1992 marked the beginning of the digital cellular network era. By 2004, GSM had over 1 billion global customers, accounting for more than 70% of the market share. GSM operates in frequency bands of 400, 800, 900, 1,800, and 1,900 MHz, with a channel bandwidth of 200 kHz. The modulation scheme is Gaussian minimum shift keying (GMSK), and the duplexing scheme is frequency division duplexing (FDD). The processing of a GSM voice call involves digitization, source encoding, channel coding, interleaving, encryption, modulation, and demodulation at the transmitter, followed by demodulation, decryption, deinterleaving, channel decoding, and source decoding at the receiver.This chapter provides an overview of 2G/2.5G and 3G wireless communication systems, with a focus on security-related aspects. Since the introduction of the first-generation cellular networks in the 1980s, wireless communications have experienced significant growth. The launch of the first commercial GSM network in 1992 marked the beginning of the digital cellular network era. By 2004, GSM had over 1 billion global customers, accounting for more than 70% of the market share. GSM operates in frequency bands of 400, 800, 900, 1,800, and 1,900 MHz, with a channel bandwidth of 200 kHz. The modulation scheme is Gaussian minimum shift keying (GMSK), and the duplexing scheme is frequency division duplexing (FDD). The processing of a GSM voice call involves digitization, source encoding, channel coding, interleaving, encryption, modulation, and demodulation at the transmitter, followed by demodulation, decryption, deinterleaving, channel decoding, and source decoding at the receiver.