This book discusses the design and evaluation of Physically Unclonable Functions (PUFs) that are suitable for securing highly energy constrained devices, while at the same time classifying the many applications and devices that would benefit from various PUF designs. The authors provide a comprehensive study of the various approaches that are used to mitigate PUFs' shortcomings, while at the same time highlighting those that are most efficient. Coverage also includes state-of-the-art approaches to designing PUFs with high resilience to machine learning attacks and hardware implementation of area-efficient and high performing PUF architectures. The authors also classify different IoT applications and devices, depending on their security needs and limitations, offering a new perspective on how different PUF designs fit in the overall scope of IoT security.