The chapter discusses the emerging low-energy frontier of particle physics, focusing on the search for weakly interacting sub-eV particles (WISPs) such as axions, axion-like particles (ALPs), and hidden U(1) gauge bosons. These particles are predicted by extensions of the Standard Model, particularly those based on supergravity or superstrings, and are expected to have very weak interactions with the visible sector. The chapter highlights the potential experimental signatures of WISPs, including their effects in experiments searching for light shining through a wall, changes in laser polarization, non-linear processes in strong electromagnetic fields, and deviations from Coulomb's law. It also reviews the current status of these searches, including constraints from astrophysics and cosmology, and discusses the advantages and challenges of low-energy experiments in detecting WISPs. The chapter concludes with a summary and outlook on the future of WISP searches.The chapter discusses the emerging low-energy frontier of particle physics, focusing on the search for weakly interacting sub-eV particles (WISPs) such as axions, axion-like particles (ALPs), and hidden U(1) gauge bosons. These particles are predicted by extensions of the Standard Model, particularly those based on supergravity or superstrings, and are expected to have very weak interactions with the visible sector. The chapter highlights the potential experimental signatures of WISPs, including their effects in experiments searching for light shining through a wall, changes in laser polarization, non-linear processes in strong electromagnetic fields, and deviations from Coulomb's law. It also reviews the current status of these searches, including constraints from astrophysics and cosmology, and discusses the advantages and challenges of low-energy experiments in detecting WISPs. The chapter concludes with a summary and outlook on the future of WISP searches.