This chapter presents a functional language micro-ML, a small subset of ML or F#. A functional programming language is one in which the evaluation of expressions and function calls is the primary means of computation. A pure functional language is one in which expressions cannot have side effects, such as changing the value of variables, or printing to the console. The micro-ML language is first-order, which means that functions cannot be used as values. Chapter 5 presents a higher-order functional language, in which functions can be used as values as in ML and F#.