This chapter introduces the concept of Physical Unclonable Functions (PUFs) as an essential element in hardware security. PUFs make use of the natural variations in integrated circuits (ICs) resulting from manufacturing processes, to provide unique and unclonable identifiers for devices. It emphasizes the significance of PUFs in device authentication and cryptographic key generation, especially in resource-constrained environments such as IoT devices. It also discusses the increasing demand for lightweight security solutions designed for devices with limited computational, memory, and power capabilities. The chapter further explains the diverse security challenges in hardware security and underscores the role of PUFs in offering secure, efficient, and tamper-resistant solutions. By integrating security directly into hardware, PUFs present a novel approach to addressing attacks that target traditional software-based security systems.