The ability to recognize the plans and goals of other agents enables humans to reason about what other people are doing, why they are doing it, and what they will do next. This fundamental cognitive capability is also critical to interpersonal interactions because human communications presuppose an ability to understand the motivations of the participants and subjects of the discussion. As the complexity of human-machine interactions increases and automated systems become more intelligent, we strive to provide computers with comparable intent-recognition capabilities.Research addressing this area is variously referred to as plan recognition, activity recognition, goal recognition, and intent recognition. This synergistic research area combines techniques from user modeling, computer vision, natural language understanding, probabilistic reasoning, and machine learning. Plan-recognition algorithms play a crucial role in a wide variety of applications including smart homes, intelligent user interfaces, personal agent assistants, human-robot interaction, and video surveillance.Plan-recognition research in computer science dates back at least 35 years; it was initially defined in a paper by Schmidt, Sridharan, and Goodson [64]. In the last ten years, significant advances have been made on this subject by researchers in artificial intelligence (AI) and related areas. These advances have been driven by three primary factors: (1) the pressing need for sophisticated and efficient planrecognition systems for a wide variety of applications; (2) the development of new algorithmic techniques in probabilistic modeling, machine learning, and optimization (combined with more powerful computers to use these techniques); and (3) our increased ability to gather data about human activities.Recent research in the field is often divided into two subareas. Activity recognition focuses on the problem of dealing directly with noisy low-level data gathered by physical sensors such as cameras, wearable sensors, and instrumented user interfaces. The primary task in this space is to discover and extract interesting patterns in noisy sensory data that can be interpreted as meaningful activities. For example, an activity-recognition system processing a sequence of video frames might start by extracting a series of motions and then will attempt to verify that they are all part of the activity of filling a tea kettle. Plan and intent recognition concentrates on identifying high-level complex goals and intents by exploiting relationships between primitive action steps that are elements of the plan. Relationships that have been investigated include causality, temporal ordering, coordination among multiple subplans (possibly involving multiple actors), and social convention. xix a form of probabilistic reasoning [12]. Charniak and Goldman [9] argued that if plan recognition is a problem of abduction, it can best be done as Bayesian (probabilistic) inference. Bayesian inference supports the preference for minimal explanations in the case of equally ...