This article provides an introduction to the field of optical orbital angular momentum (OAM), a topic that has seen significant development over the past three decades. The authors, Stephen M. Barnett, Mohamed Babiker, and Miles J. Padgett, highlight the contributions of Les Allen, who was a key figure in the early stages of this research. The article covers the historical context, theoretical foundations, and practical applications of OAM. Key points include: 1. **Introduction to OAM**: The field of OAM, which involves the study of angular momentum about the axis of light propagation, has attracted researchers from various backgrounds. The term "OAM" can refer to either orbital or optical angular momentum, though the distinction is not universally agreed upon. 2. **Theoretical Background**: The article explains the quantum mechanical description of OAM, including the separation of angular momentum into spin and orbital parts. It also discusses the paraxial approximation and the use of Laguerre-Gaussian beams, which are known for their phase vortices and associated orbital angular momentum. 3. **Early Developments (1992–2005)**: The initial discovery that laser beams could carry well-defined OAM led to rapid theoretical and experimental advancements. This included the use of cylindrical lenses to generate such beams, the study of torque on atoms in OAM fields, and the demonstration of OAM in optical tweezers and entangled photon pairs. 4. **Recent Developments**: Since 2005, OAM has gained widespread interest, with numerous conferences and publications dedicated to the topic. The special issue includes contributions from leading researchers, covering a wide range of topics such as beam generation, practical applications, communication, and interactions with matter. 5. **Conclusion**: The article concludes with a tribute to Les Allen, highlighting his significant contributions to the field and his role in fostering interest and research in OAM. The special issue is dedicated to Allen and aims to celebrate 25 years of research in this area. Overall, the article serves as a comprehensive overview of the field of optical orbital angular momentum, emphasizing its theoretical foundations, experimental advancements, and practical applications.This article provides an introduction to the field of optical orbital angular momentum (OAM), a topic that has seen significant development over the past three decades. The authors, Stephen M. Barnett, Mohamed Babiker, and Miles J. Padgett, highlight the contributions of Les Allen, who was a key figure in the early stages of this research. The article covers the historical context, theoretical foundations, and practical applications of OAM. Key points include: 1. **Introduction to OAM**: The field of OAM, which involves the study of angular momentum about the axis of light propagation, has attracted researchers from various backgrounds. The term "OAM" can refer to either orbital or optical angular momentum, though the distinction is not universally agreed upon. 2. **Theoretical Background**: The article explains the quantum mechanical description of OAM, including the separation of angular momentum into spin and orbital parts. It also discusses the paraxial approximation and the use of Laguerre-Gaussian beams, which are known for their phase vortices and associated orbital angular momentum. 3. **Early Developments (1992–2005)**: The initial discovery that laser beams could carry well-defined OAM led to rapid theoretical and experimental advancements. This included the use of cylindrical lenses to generate such beams, the study of torque on atoms in OAM fields, and the demonstration of OAM in optical tweezers and entangled photon pairs. 4. **Recent Developments**: Since 2005, OAM has gained widespread interest, with numerous conferences and publications dedicated to the topic. The special issue includes contributions from leading researchers, covering a wide range of topics such as beam generation, practical applications, communication, and interactions with matter. 5. **Conclusion**: The article concludes with a tribute to Les Allen, highlighting his significant contributions to the field and his role in fostering interest and research in OAM. The special issue is dedicated to Allen and aims to celebrate 25 years of research in this area. Overall, the article serves as a comprehensive overview of the field of optical orbital angular momentum, emphasizing its theoretical foundations, experimental advancements, and practical applications.