Thin polymer films that are able to bind proteins are of great relevance for a wide variety of biosensor and biochip applications. Some of these applications require films that have high protein binding capacities while others call for surfaces that allow to control the orientation and preserve the biological activity of the immobilized proteins. Designing protein binding polymer interfaces with properties that are tailored for a specific biosensor or biochip configuration is a creative challenge that provides a host of possibilities to put the assets of modern controlled radical polymerization techniques in practical, technological use. For the preparation of protein binding polymer interfaces, surface-initiated controlled radical polymerization techniques are very attractive since they allow good control over the molecular weight, architecture, grafting density and functionality of the resulting surface-attached polymer assemblies, which are typically referred to as polymer brushes. This review article presents an overview of polymer brush films prepared via surface-initiated controlled radical polymerization, which have been designed as protein binding interfaces. The focus of this article will be on the protein immobilization chemistries that have been explored and the underlying polymer chemistry that is needed to generate these films and control their properties. For the immobilization of proteins on polymer brush films, two principal approaches can be followed; (i) covalent and (ii) non-covalent immobilization. For each of these main approaches an overview of the different polymer brush chemistries will be presented that can be used to covalently or non-covalently bind proteins.