Mathematica is a software program that provides the means for solving mathematical problems commonly faced by scientists, engineers, and mathematicians. This guide is designed to help professionals use Mathematica to simplify such tasks as finding appropriate commands, manipulating expressions, visualizing functions and data, writing functions and packages, and importing and exporting data. Though written with the new user in mind, it contains information of interest to those who have experience with the program, including research scientists and engineers and mathematicians.