This study characterizes the interaction of asphalt with Crumb Rubber Modifier (CRM) using performance testing as used by SHRP. The study provides an understanding of the property development as asphalt interacted with CRM. Two main types of activities are discussed in the paper: particle swelling and depolymerization. Two types of interactions are considered: a short-term interaction that explains the property development mechanism and a long-term interaction where the property stabilizes. The effect of CRM properties on the developed properties is discussed in details.