Explore finite-size effects in integrable systems through this comprehensive lecture by Fabian Essler from the International Centre for Theoretical Sciences. Delve into topics such as relaxation of time averages, the diagonal ensemble, traversals and revivals in finite systems, and macro states in free theories. Examine conservation laws in interacting integrable models, including the string hypothesis and composition principle. Learn about energy eigenstates, occupation numbers, and particle/hole densities in the context of integrable systems. Gain insights into the thermodynamic entropy and generalized micro-canonical ensemble. The lecture concludes with a Q&A session, providing an opportunity to deepen understanding of these advanced concepts in mathematical physics and condensed matter theory.