Identification, authentication, and integrity checking are important tasks for ensuring the security and protection of valuable objects, devices, programs, and data. The utilization of the microscopic, random and unclonable disorder of physical media for such security tasks has recently gained increasing attention. Wherever applicable, the harnessing of disorder can lead to intriguing advantages: First, it can avoid the permanent storage of digital secret keys in vulnerable hardware, promising to make the resulting systems more resilient against invasive and malware attacks. Second, random physical disorder has the natural feature of being very hard to clone and to forge: Fully controlling the microand nanoscale fabrication variations in physical media is extremely difficult and, even if possible, prohibitively expensive. Third, utilization of the natural disorder and entropy in physical systems can sometimes enable cryptographic protocols whose security does not rest on the usual unproven number-theoretic assumptions like factoring and discrete log, creating an alternate foundation for cryptography. Physical Unclonable Functions or PUFs are perhaps the best known representative of this new class of “disordered” cryptoprimitives, but there are also others. In this chapter, we provide a classification for past and ongoing work in physical disorder based security alongside with security analyses and implementation examples. We will also outline some open problems and future research opportunities in the area.