Presented by Yu Hang Ng: I will start the review by briefly describing the strong CP problem, and explaining how it can be solved by introducing a global U(1) symmetry that is known as Peccei-Quinn symmetry. Axion is the Nambu-Goldstone boson corresponds to the spontaneously broken Peccei-Quinn symmetry. Then, two popular axion models (Kim-Shifman-Vainstein-Zakharov (KSVZ) model and Dine-Fischler-Srednicki-Zhitnitskii (DFSZ) model) will be introduced. These models predict very light axions that couple very weakly with Standard Model particles. Nonetheless, there are several classes of experiments that can probe the parameter space of these very light axions. These experiments can also be used to search for a class of very light pseudoscalar particles that is known as the axion-like particles (ALPs). Short descriptions about these experiments will be given.