Young stars are surrounded by circumstellar disks for several million years; it is in these disks that planets are thought to form. The Spitzer Space Telescope has been providing a wealth of data that aids in understanding the evolution of protoplanetary disks. I will present results from Spitzer that reveal significant disk evolution, ranging from dust grain growth to disk clearing, already at an age of 1 million years. Thus, we are observing the first steps of planet formation in several of these young disks, which place constraints on planet formation mechanisms.