The development of surfaces with well-engineered properties is actually a significant challenge for chemists and physicists. For example, surfaces and interfaces responsive to the surrounding environment were recently identified as essential in the design of innovative devices, from electronics to medicine. In this perspective, polymer brushes, which consist of thin organic layers of polymers tethered by one end to a substrate, are an extremely versatile tool in surface engineering. Several approaches were developed to control the surface characteristics. Among these, polymer brush technology based on the “grafting to” reaction has been widely applied. In the “grafting to” process, a functional polymer is grafted onto the substrate and good control over the brush characteristics is obtained because of the self-limiting nature of this reaction and the possibility to tune the number of chains by changing the molecular weight of the employed polymer. Furthermore, the polymeric materials suitable for the “grafting to” process are prepared by the quite sophisticated chemistry of controlled or living polymerizations. However, recent investigations revealed that the “grafting to” reaction is much more complex than usually believed with a shift from a diffusion-controlled to a mechanochemical driven mechanism. Moreover, partitioning by molecular weight takes place at interfaces in which the lower molecular weight species are preferably incorporated into the polymeric brush. All these aspects will be discussed to highlight what is true, what is false and what is still not clear in this process.