Structure101 is a tool to understand, measure, and control software structure. It allows you to see exactly how your high-level components depend on each other and why. You can use it to find out immediately when your architecture is accidentally changed by code-level changes at the coal-face. It can help you control structural complexity, since it can measure the complexity of methods, classes, and packages and warn you when given limits are exceeded. It can also discover the locations of productivity-killing package dependency cycles.