This paper summarizes a series of research studies conducted over a period of six years in which we investigated the effectiveness of virtual environments for aircraft maintenance training. In these studies, we systematically evaluated numerous aspects of virtual maintenance trainers (VMTs) that may impact training effectiveness, including comparisons of immersive vs. desktop virtual environments; high vs. low detail graphics; task selection and specific tasking requirements; and the effects of individual differences on learning effectiveness within virtual environments. In each study, we conducted an evaluation of training effectiveness, collecting objective performance measures of declarative knowledge, training transfer task performance time, and transfer performance errors. In addition, individual difference measures of spatial reasoning aptitude, computer/video game experience, and hand tool experience afforded the opportunity to analyze the impacts of these variables on VMT effectiveness. Within each study, rigorous experimental protocol in training procedures and data collection remained constant, allowing us to compare results across the multiple studies. The paper highlights significant results from each of these individual studies, as well as generalized findings across all of the studies. Our studies showed that interactivity and high graphic detail are important for training effectiveness, and that desktop VE trainers were a significantly more effective medium than immersive training. In addition, individual spatial reasoning aptitude is a significant mediator of virtual training effectiveness when the task would be learned and performed with limited visual feedback in the real world (e.g., a tactile or "blind" task). The impact of the spatial processing aptitude is similar for both hardware and VE-based training, however, when the task is normally learned and performed in the visual field. This paper summarizes these findings, as well as other study results, as a series of empirically driven guidelines for the implementation of virtual environments for aircraft maintenance training.