Disclosed are composites that can exhibit low transmission energy loss and can also be temperature resistant. The composites include reinforcement fibers held in a polymeric matrix. The reinforcement fibers can include an amorphous polymer component. The fibers can be woven or knit to form a fabric or can be included in a nonwoven fabric. The composites can include other fibers as well, such as fiberglass. The composites can be multi-layer structures and can include layers of other materials, for instance layers formed of polyaramids, fiberglass, or carbon fiber wovens or nonwovens. The composites can advantageously be utilized in low loss dielectric applications, such as in forming circuit board substrates.