As a future engineer, one of the most fundamental skills you will develop is the ability to make smart economic decisions. From choosing the right materials for a bridge to helping launch a new product, your choices will have significant financial consequences. To make the best decisions, it’s crucial to understand that “cost” isn’t a single, simple number. Instead, it’s a concept with many different facets. Learning to see a project through the lens of different types of costs—fixed, variable, marginal, and opportunity—is a core skill that transforms a good engineer into a great one. This guide will introduce you to these essential building blocks of economic analysis.