To assist in making difficult decisions about the use of fillers and fibers in polyolefins, this chapter contains a number of case studies describing actual uses of the additives covered in Chapter 7. Case applications include pipe extrusion, blow molding, polyethylene film, polypropylene tapes, thermoplastic olefins, and short and long glass fiber compounds. The cases are linked to major points about the use of filers, fibers, and impact modifiers. Essential points are covered concisely, relating the additives’ economics issues, property interactions, and processing challenges. Also covered are their interactions with other types of additives, and unexpected complications (pitfalls) encountered when using them that have to do with screw processing, handling hazards, processing effects on properties, and recyclability.