This article reviews complementary metal-oxide-semiconductor (CMOS) based physically unclonable functions (PUFs) in terms of types, structures, metrics, and challenges. The article reviews and classifies the most basic PUF types. The article reviews the basic variations originated during a metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process. Random variations at transistor level lead to acquiring unique properties for electronic chips. These variations help a PUF system to generate a unique response. This article discusses various concepts which allow for more variations at CMOS technology, layout, masking, and design levels. It also discusses various PUF related topics.