In this review, we summarize the latest studies on thermal conductivity (TC) of composite materials. TC enhancement of composite materials has been manipulated by using fillers of different properties: material type, size, geometries, and functionalization. Examples of filler size ranges are nanoparticles (1-100 nm) and microparticles (>100 nm); filler geometries could be tubes or sheets, whereas filler materials could be carbon or boron nitride, which will be the focus of this review. It has been found that hybrid composite materials, i.e., systems containing more than one type of filler, such as a blend of nanosize and microsize fillers, give better TC than any of these individual fillers. Besides the filler intrinsic properties, its concentration, dispersion quality, and the preparation method of the composite affects the composite properties as well. The challenge is to optimize these parameters with the desired final performances.