Fiber-reinforced composites and textile filters are being increasingly used in our daily lives. Their performances strongly depend on the fibers' surface properties. A high-performance fiber gives its strength to a composite through the interface between the matrix and the fiber surface. The particles to be filtered are also collected on the fiber surface. However, in some cases fiber surface characteristics are not very suitable for good adhesion or high rates of particle collection. There are several methods for surface modification of fibers. In this chapter the surface properties of reinforcing and filtering fibers are described and the surface modifications for special applications are explained.