Physical Unclonable Functions (PUFs) are circuit primitives that generate chip specific and unique outputs, depending on the uncontrollable variations present in the manufacturing process. These cheap and highly efficient structures have a wide range of application areas, including authentication, key generation, and IP protection. Uniqueness, robustness and unpredictability are other important aspects of PUF circuits beside unclonability. In this work, we first review basic PUF circuit types, including Optical PUFs, Arbiter PUFs, Ring Oscillator (RO) PUFs and, SRAM PUFs. Then, two FPGA implementations of RO-PUFs are presented with their uniqueness and robustness analyses. Finally, new concepts in RO-PUF literature and their limits and performance expectations are discussed.