This lecture covers the concept of asymptotic states and S-matrix in quantum field theory, focusing on the Lippmann-Schwinger equation. The instructor explains the formalization of states in an interacting theory and the process of defining complete sets of states. The lecture also delves into the explicit solution for the Lippmann-Schwinger equation, emphasizing the importance of understanding the asymptotic behavior of states. Additionally, the lecture discusses the Möller operators and the notion of energy robustness in the context of quantum field theory.