In this chapter, we first introduce entropy and some basic concepts related to information theory. We show how entropy has find its way from thermodynamics into information theory and consequently into related areas including cryptography. Then, we show and investigate the role of entropy and information theory in the ecosystem of cryptography. Next, we study perfect secrecy along with its notions, approaches, and variants. We continue to study the applications of perfect secrecy in cryptography and related scenarios. In the next step, we present an overview on OTP as the only real-world implementation of perfect secrecy and describe its role in the future of cryptography.