React Developer Tools is a Chrome extension that allows you to inspect the React component hierarchy in the Chrome Developer Tools (formerly WebKit Web Inspector).
This extension adds a new tab called React in your Chrome DevTools. This shows you the root React components that was rendered on the page, as well as the subcomponents that they ended up rendering.
By selecting one of the components in the tree you can inspect and edit its current props and state in the panel on the right. In the breadcrumbs you can inspect the selected Component, the Component that created it, the Component that created that one, and so on.