Bilateral arm training has emerged as an approach that leads to positive outcomes in addressing upper extremity paresis after stroke. However, studies have not demonstrated improvements in all patients using current outcome measures. Furthermore, the rationale for using this type of training has been incompletely explained. The purpose of this article was to first review the theoretical justifications for the use of bilateral arm training by examining motor control and neural mechanisms underlying arm function and neural recovery, and second, to discuss examples of clinical studies using a variety of bilateral training strategies to identify who may benefit most from this approach. We argue that bilateral arm training is a necessary adjunct to unilateral training because bilateral re-training is important and best served through bilateral not unilateral training, and also, that bilateral training may help unilateral skill recovery through alternative putative mechanisms. Our review of the empirical evidence suggests that individuals at all levels of severity can benefit in some manner from bilateral training, but that not all approaches are effective for all severity levels. In addition to requesting more randomized controlled trials and studies of neurophysiological mechanisms we conclude the following: 1) Bilateral training can improve unilateral paretic limb functions of the upper extremity after stroke, however, specific training approaches need to be matched to baseline characteristics of the patients; 2) Given the importance of bilateral activities in daily life, there is a need to recognize, train and assess the important contribution of supportive role functions of the paretic arm used on its own and as part of complementary bilateral functional skills; 3) An assessment of bilateral and unilateral functioning which includes bilateral task analysis, as well as, evaluations of interlimb coordination should be included in all studies that include bilateral training; 4) Studies with thoughtful sequencing or combining of bilateral approaches or sequencing of bilateral and unilateral approaches are needed to assess if there are improved outcomes in paretic and bilateral limb function.