The performance of a variety of materials added to asphalt binders and mixtures to change their properties, particularly with respect to rutting and cracking, was investigated. Two approaches were taken. The first was a field trial of seven modifiers. The modifiers evaluated included plastomers, elastomers, chemical modifiers, and particulates. The second approach was a laboratory study of the effects of these modifiers on binder and mixture properties. The field trial showed that modifiers are not essential to ensure that the pavement will not rut; none of the mixtures evaluated here exhibited appreciable rutting. Dramatic differences were noted in the cracking behavior, however. The newly developed MP-1a binder tests were able to identify binders that were more prone to cracking.