This report documents the Lincoln Laboratory evaluation of the Traffic Alert and Collision Avoidance System II (TCAS II) logic version 6.04a. TCAS II is an airborne collision avoidance system required since 30 December 1993 by the FAA on all air carrier aircraft with more than 30 passenger seats operating in U.S. airspace. Version 6.04a is a logic version mandated by the FAA by 30 December 1994 in order to correct a potential safety problem in earlier versions and to make the TCAS logic more compatible with the air traffic control system. Lincoln Laboratory evaluated the logic by examining approximately two million simulated pairwise TCAS-TCAS encounters, derived from actual aircraft tracks recorded in U.S. airspace. The main goals of the evaluation effort were: (1) to determine if version 6.04a successfully corrected the potential safety problem without introducing new problems; (2) to detect and explain any areas of poor performance; and (3) to understand the performance limits of the logic. Five analysis programs were written to aid in the evaluation, and these programs are described in the report. There were three phases of the evaluation corresponding to the above three goals. For each phase, the report gives an overview of the evaluation approach taken, a description of the results, and a summary. A description of follow-on activities plus overall conclusions and recommendations are given at the end of the report.