Assertions (or expectations) are the most important part of any test framework. How are they written? What happens when one fails? How does a test communicate its results? Past talks have shown how test frameworks work from the very top: how they find, load, select, and run tests. Instead of reading code from the top, we’ll write code from scratch starting with assertions and building up a full test framework. By the end, you’ll know how every square inch of your testing framework works.