This article defines a category of human-robot interaction in which human and robotic actors work as a single unified system. We survey work from various fields including human augmentation systems such as extra fingers and arms, and other robots that operate in close proximity to the user. The discussed works highlight a close interplay between human and robotic actions where control decisions are made by both actors. Such a dyadic configuration can yield a synergistic outcome but require that close attention be paid to the coordination between them. Using case studies from our own work, we discuss two main questions that must be addressed when designing such closely collaborative human-robot integrations: type of support and degree of control. The different choices that can be adopted for each of these design questions define a framework or classification that is useful for surveying existing and future research.