A method of forming a radius filler for a composite structure, wherein the composite structure includes a plurality of plies of composite material that converge within a transition region to define an elongate void space, and further wherein the radius filler is configured to extend within the elongate void space, the method comprising: determining a transverse cross-sectional shape of the elongate void space; determining a material property field of the transition region; and forming the radius filler, wherein the forming includes: combining a plurality of lengths of composite tape to define a radius filler transverse cross-sectional shape that corresponds to the transverse cross-sectional shape of the elongate void space, wherein each of the plurality of lengths of composite tape includes a respective plurality of lengths of reinforcing fiber and a resin material, and further wherein each of the plurality of lengths of composite tape defines a fiber axis direction; and orienting the plurality of lengths of reinforcing fiber of each of the plurality of lengths of composite tape based, at least in part, on the material property field of the transition region, wherein the determining the material property field of the transition region includes determining a principal stress region within the transition region and determining a principal stress direction within the principal stress region, and further wherein the orienting includes orienting such that at least one of the plurality of lengths of composite tape defines a respective fiber axis direction that extends within a portion of the radius filler that is located within the principal stress region and at least substantially parallel to the principal stress direction. determining a transverse cross-sectional shape of the elongate void space; determining a material property field of the transition region; and forming the radius filler, wherein the forming includes: combining a plurality of lengths of composite tape to define a radius filler transverse cross-sectional shape that corresponds to the transverse cross-sectional shape of the elongate void space, wherein each of the plurality of lengths of composite tape includes a respective plurality of lengths of reinforcing fiber and a resin material, and further wherein each of the plurality of lengths of composite tape defines a fiber axis direction; and orienting the plurality of lengths of reinforcing fiber of each of the plurality of lengths of composite tape based, at least in part, on the material property field of the transition region, wherein the determining the material property field of the transition region includes determining a principal stress region within the transition region and determining a principal stress direction within the principal stress region, and further wherein the orienting includes orienting such that at least one of the plurality of lengths of composite tape defines a respective fiber axis direction that extends within a portion of the radius filler that is located within the principal stress region and at least substantially parallel to the principal stress direction. combining a plurality of lengths of composite tape to define a radius filler transverse cross-sectional shape that corresponds to the transverse cross-sectional shape of the elongate void space, wherein each of the plurality of lengths of composite tape includes a respective plurality of lengths of reinforcing fiber and a resin material, and further wherein each of the plurality of lengths of composite tape defines a fiber axis direction; and orienting the plurality of lengths of reinforcing fiber of each of the plurality of lengths of composite tape based, at least in part, on the material property field of the transition region, wherein the determining the material property field of the transition region includes determining a principal stress region within the transition region and determining a principal stress direction within the principal stress region, and further wherein the orienting includes orienting such that at least one of the plurality of lengths of composite tape defines a respective fiber axis direction that extends within a portion of the radius filler that is located within the principal stress region and at least substantially parallel to the principal stress direction.