State-of-the-art technology is available which incorporates shredded scrap tire rubber known as crumb rubber modifier (CRM) as a modifier for asphalt cement in paving mixtures. CRM is the general term to describe all rubber particles obtained from the shredded of discarded tires which are reduced up to a size of 6.35 mm. Two technologies are most commonly used to incorporate crumb rubber in asphalt pavement applications, namely, the wet and dry process. The wet process, also known as the modified binder method, covers any method that blends the crumb rubber with the asphalt cement (AC) prior to incorporating the binder. The dry process applies to technologies that mix the crumb rubber with the aggregate prior to being charged with the asphalt binder. In 1991, the Intermodal Surface Transportation Efficiency Act (ISTEA), Section 1038, mandated the use of CRM in asphalt paving mixtures for all states including Puerto Rico. This report describes the first major research effort conducted at the University of Puerto Rico at Mayagez (UPRM) to characterize hot mix asphalt (HMA) paving mixtures using crumb rubber as a modifier.