What is boredom? We review environmental, attentional, and functional theories and present a new model that describes boredom as an affective indicator of unsuccessful attentional engagement in valued goal-congruent activity. According to the Meaning and Attentional Components (MAC) model, boredom is the result of (a) an attentional component, namely mismatches between cognitive demands and available mental resources, and (b) a meaning component, namely mismatches between activities and valued goals (or the absence of valued goals altogether). We present empirical support for four novel predictions made by the model: (a) Deficits in attention and meaning each produce boredom independently of the other; (b) there are different profiles of boredom that result from specific deficits in attention and meaning; (c) boredom results from two types of attentional deficits, understimulation and overstimulation; and (d) the model explains not only when and why people become bored with external activities, but also when and why people become bored with their own thoughts. We discuss further implications of the model, such as when boredom motivates people to seek interesting versus enjoyable activities.