Ti-5Al-5Mo-5V-3Cr-1Fe (TIMETAL-555 or Ti-555) is one of the more recently developed beta Titanium alloys and consequently the microstructural evolution and resultant mechanical properties of this alloy have not yet been explored in great detail. As-received, alpha+beta processed Ti-555, was subjected to a number of heat treatments with the objective of studying the microstructure and phase evolution in this alloy and obtain widely varying microstructures. The typical heat-treatment consisted of solutionization and recrystallization in the beta phase field, cooling from above beta transus temperature at different rates, and a second aging treatment in the alpha+beta phase field. The microstructure at each stage of the heat-treatment cycle has been characterized in detail using SEM and TEM based studies and these results will be discussed in this paper. The nucleation and growth of the alpha phase in the beta matrix will be specifically addressed. In addition, the results of studies on microstructural evolution in alloys solutionized and aged in the alpha+beta phase field will also be discussed. Finally, preliminary data on the mechanical properties associated with these different microstructures in Ti-555 will be presented.