This lecture covers the Moore-Penrose pseudoinverse, a generalised inverse for arbitrary matrices. It explains its existence, uniqueness, and properties, such as (P1) AGA = A, (P2) GAG = G, (P3) (AG)^T = AG, and (P4) (GA)^T = GA. The lecture also discusses different characterizations of the pseudoinverse and its applications in systems of linear equations and least squares problems.