Yao and Padgett's article, "Orbital angular momentum: origins, behavior and applications," discusses the concept of orbital angular momentum (OAM) in light beams. Unlike spin angular momentum (SAM), which is associated with the polarization of light, OAM arises from the phase structure of a beam. Light beams with a helical phase front, described by an azimuthal phase dependence of exp(iℓφ), carry OAM, which can be much greater than SAM. The article traces the history of OAM, starting with Allen et al.'s 1992 discovery that such beams can be easily generated in the lab. It covers the generation of helically phased beams using methods like spiral phase plates, Laguerre-Gaussian modes, and diffractive optical elements. The interaction of these beams with matter is explored, including their ability to spin microscopic objects, create rotational frequency shifts, and interact with cold atoms. The article also discusses applications in nonlinear and quantum optics, such as optical vortices in Kerr media, parametric down conversion, and quantum entanglement. Techniques for measuring OAM, such as forked diffraction gratings and interferometry, are described. The article highlights the significance of OAM in various fields, from micromachines to imaging and quantum communication, and emphasizes its potential for future research and technological advancements.Yao and Padgett's article, "Orbital angular momentum: origins, behavior and applications," discusses the concept of orbital angular momentum (OAM) in light beams. Unlike spin angular momentum (SAM), which is associated with the polarization of light, OAM arises from the phase structure of a beam. Light beams with a helical phase front, described by an azimuthal phase dependence of exp(iℓφ), carry OAM, which can be much greater than SAM. The article traces the history of OAM, starting with Allen et al.'s 1992 discovery that such beams can be easily generated in the lab. It covers the generation of helically phased beams using methods like spiral phase plates, Laguerre-Gaussian modes, and diffractive optical elements. The interaction of these beams with matter is explored, including their ability to spin microscopic objects, create rotational frequency shifts, and interact with cold atoms. The article also discusses applications in nonlinear and quantum optics, such as optical vortices in Kerr media, parametric down conversion, and quantum entanglement. Techniques for measuring OAM, such as forked diffraction gratings and interferometry, are described. The article highlights the significance of OAM in various fields, from micromachines to imaging and quantum communication, and emphasizes its potential for future research and technological advancements.