This chapter focuses on the unique security challenges that resource-constrained IoT devices, like RFID systems, wearables, and medical implants, face. It looks at these devices’ limitations in terms of power, memory, and computational capabilities and how these constraints affect their ability to use traditional cryptographic methods. The chapter proposes solutions based on Physically Unclonable Functions (PUFs) as lightweight alternatives to ensure these devices’ secure authentication and operation in exposed environments.