A challenging introduction to programming languages and computer science that emphasizes alternative modes of algorithmic expression. Topics include recursive and higher-order procedures, performance analysis of algorithms, proof of program correctness, probabilistic algorithms, symbolic hierarchical data, abstract data types, polymorphic functions, object oriented programming, infinite data types, simulation, and the interpretation of programs. It is recommended that students planning to major in Computer Science take Computer Science 18 rather than Computer Science 15 or 19.