Explore the fascinating realm of dark energy and modified gravity in this comprehensive lecture by Alessandra Silvestri from the International Centre for Theoretical Sciences. Delve into the complexities of our universe, starting with an overview of the ΛCDM model and the uniqueness of General Relativity. Examine various theoretical frameworks, including scalar-tensor theories, f(R) gravity, Galileons, and Horndeski theory. Learn about the impact of gravitational wave observations, particularly GW170817, on modified gravity theories. Investigate screening mechanisms and their types, with a focus on the Chameleon Mechanism. Gain insights into local gravity tests, theory stability, and observable effects of modified gravity. Conclude with a Q&A session for deeper understanding of these cutting-edge concepts in cosmology and gravitational physics.