This lecture covers the Hamilton-Jacobi equation, focusing on the phase portrait analysis. It explains the ambiguity of the equation and how to write it explicitly. The lecture delves into defining positions, angles, and energy-action relationships in phase portraits. It also discusses one-dimensional systems, phase space, and the evolution of systems in phase space. The instructor illustrates examples of Poincaré sections and chaotic systems, emphasizing the relationship between variables and the system's evolution.