Explore the fundamentals of gravitational lensing in this comprehensive lecture by Alan Heavens, part of the "Cosmology - The Next Decade" program at the International Centre for Theoretical Sciences. Delve into the fascinating phenomenon of light bending due to massive objects in space, a key concept in modern cosmology. Learn how gravitational lensing serves as a powerful tool for studying dark matter, galaxy clusters, and the large-scale structure of the universe. Gain insights into the mathematical framework and observational techniques used to detect and analyze gravitational lensing effects. This 52-minute lecture provides a solid foundation for understanding one of the most important topics in contemporary astrophysics and cosmology.