In the spring of 1976, George Andrews discovered a sheaf of 138 pages in the handwriting of Srinivasa Ramanujan. This manuscript was soon designated “Ramanujan's lost notebook” Its discovery has frequently been deemed the mathematical equivalent of finding Beethoven's tenth symphony. The “lost notebook” contains considerable material on mock theta functions and so undoubtedly emanates from the last year of Ramanujan's life. It should be emphasized that the material on mock theta functions is perhaps Ramanujan's deepest work.