This chapter discusses the security threats faced by Physically Unclonable Functions (PUFs), including machine learning attacks, side-channel attacks, and environmental manipulations. It explores the vulnerabilities that result from PUFs relying on physical properties and how attackers exploit these characteristics to compromise device security. It also covers techniques for mitigating these threats, such as fault injection attacks and hardware Trojan detection, and emphasizes ongoing research efforts to enhance PUF robustness against sophisticated adversaries.