Mixed polymer brushes as functional ultra thin films for surface functionalization have an enormous potential to create a variety of smart, switchable, and multifunctional surfaces and thin films. It is shown how computer simulations can contribute to a better understanding of the switching behavior of brushes. Furthermore, it is described how polymer brushes can be used to create surfaces with switchable ultrahydrophobicity and wettability gradients, as well as functional layers for the immobilization of nanoparticles. Applications of these versatile and multifunctional brush coatings are envisioned in many areas including fluid control, microfluidics, and thin film sensors.