This book studies the security of Physical Unclonable Functions (PUFs) based on the Arbiter PUF design, with a focus on so-called modeling attacks. After revisiting modeling attacks on the Arbiter PUF and XOR Arbiter PUF, the author demonstrates attacks against the Lightweight Secure XOR Arbiter PUF, Feed-Forward Arbiter PUF, and the Interpose PUF.