The corrosion of reinforcing steel is a significant and preventable problem that shortens the useful lifespan of concrete structures. The mechanisms for its development and how it eventually damages the concrete structures it was designed to protect have been well-known for over 100 years. Similar to progressive disease, early detection and monitoring are the most effective strategies to minimize problems. Fortunately, innovative nondestructive instruments are making it easier and more efficient to find, follow, and repair corrosion before extensive damage occurs.