Although there has been great speculation about the potential of augmented reality (AR) in manufacturing applications, there have been very few empirical studies that assess the effectiveness of the technology. This chapter describes an experiment that tested the relative effectiveness of AR instructions in computer-assisted assembly. The AR solution displayed task information in the user’s field of view (FOV) as 3D objects registered with the workspace. The presentation demonstrates the exact execution of a procedural step. Three traditional instructional media approaches were compared with the AR system: a printed manual, computer-assisted instruction (CAI) using a monitor-based display, and CAI utilizing a head-mounted display (HMD). Results indicate that overlaying 3D instructions on the actual work pieces reduced the error rate for an assembly task by 82%, particularly diminishing cumulative errors, errors due to previous assembly mistakes. Measurement of the mental effort indicated decreased mental effort in the AR condition, suggesting some of the mental calculation of the assembly task is offloaded to the system. The results indicate that an AR system for computer-assisted assembly can improve worker performance.