Stream cipher security is heavily dependent on the quality of the pseudo-random number generators (PRNGs) used to generate key streams. While true randomness is ideal, real-world implementations must rely on deterministic algorithms, making the development of high-quality PRNGs essential. This chapter explores the critical attributes of secure generators, such as unpredictability, efficiency, and statistical robustness. It compares PRNGs designed for computational simulations with those intended for cryptographic security, exposing the vulnerabilities of linear generators. The discussion covers key PRNGs, including LFSRs, MRGs, and MT19937, along with cryptographic alternatives like DRBGs, examining their strengths, weaknesses, and practical applications.