Since the first report on blue electroluminescence from a soluble poly(9,9-di-alkylfluorene), fluorene-based homo- and copolymers have evolved as a major class of polymeric emitters for highly efficient organic light-emitting diodes. This Review is concerned with the particular properties of soluble derivatives of polyfluorene homopolymers with respect to emission properties, control of color stability and efficiency in electroluminescence, alignment in thin layers and polarized emission. A major point of discussion is the origin of unwanted emission contributions in the photoluminescence and electroluminescence spectra of solid layers as well as concepts to avoid these contributions in order to ensure stable device performance. Further, the alignment of polyfluorenes and their use in polarized light-emitting diodes is addressed.