This document is the first of seven units developed by the Math Network Curriculum Project. Each unit, designed to be a 2-week module, is a teacher's guide which includes detailed directions along with the courseware and software needed. Teacher intervention in the non-computer activities that begin each unit is required, and the consistent use of small-group instruction makes the units usable in a standard classroom if two microcomputers are present. The Input-Output Unit develops students' abilities to recognize and describe patterns and introduces the notion of expressing mathematical concepts in a symbolic language, Easy Speak. The experience of using Easy Speak prepares students to use algebra to express numerical ideas. Practice is provided on simple input-output machines, many of which grow out of activities with manipulative materials, and moves to machines with two-part rules. Printed copies of the code for the Wizard and Easy Speak computer programs are included. Wizard reinforces the idea of conditional statements by having students create their own and interact with the computer. Easy Speak, used with Easy Speak Summary, permits students working in pairs to create mystery machines for pairs of numbers. They can also try mystery machines created by students in other classes. Both programs were developed for use on a Commodore PET Computer with at least 16K of RAM using 4.0 BASIC. (MNS)