Radius fillers for composite structures, composite structures that include the radius fillers, and systems and methods of forming the radius fillers. The systems and methods may be configured to create a pre-form that may be utilized to form a radius filler and/or to create a radius filler from the pre-form. The systems may include an apparatus configured to form the pre-form for the radius filler. This apparatus includes a composite tape source, a pre-forming structure, and a drive mechanism. The systems also may include an apparatus configured to form the radius filler. This apparatus includes a pre-form supply, a first radius filler die, a second radius filler die, and a die selection structure.