I will address the basic question of how much a person can think about at one time, and how the contents of the conscious mind might be measured. In 1956 George A. Miller, a founding cognitive psychologist who died recently, suggested that the amount of information that one can recall from a just-presented list is fixed at about seven meaningful units, or chunks. Two critically important issues about this working memory capacity limit were left unresolved, however: (1) how one can reliably measure meaningful units, and (2) how much of the information is retrieved from the conscious mind as opposed to a more automatic type of memory. Our laboratory has advanced the understanding of these issues in recent years using a combination of methods including dual-task experimentation, brain imaging, mathematical modeling, and child developmental research. I will review the evidence and suggest that the mental faculty that is limited in capacity, to about 3 or 4 chunks in normal adults, is the focus of attention.