Book excerpt: With CMOS scaling extending transistors to nanometer regime, process variations from manufacturing impacts modern IC design. Fortunately, such variations have enabled an emerging hardware security primitive - Physically Unclonable Function. Physically Unclonable Functions (PUFs) are hardware primitives which utilize disorder from manufacturing variations for their core functionality. In contrast to insecure non-volatile key based roots-of-trust, PUFs promise a favorable feature - no attacker, not even the PUF manufacturer can clone the disorder and any attempt at invasive attack will upset that disorder. Despite a decade of research, certain practical problems impede the widespread adoption of PUFs. This dissertation addresses the important problems of (i) post-manufacturing testing, (ii) secure design and (iii) cost efficiency of PUFs. This is with the aim of making PUFs practical and also learning hardware design limitations of disorder based systems.