Computational models are mostly process-based theories, that is, they are mostly directed at answering the question of how human performance comes about; by what psychological mechanisms, processes, 1Therootsofcognitivesciencecan,ofcourse,be traced back to much earlier times. For example, Newell and Simon’s early work in the 1960s and 1970s has been seminal (see, e.g., Newell & Simon, 1976). The work of Miller, Galanter, and Pribram (1960) has also been highly influential. See Chap- ter 25 in this volume for a more complete historical perspective (see also Boden, 2006). and knowledge structures; and in what ways exactly. In this regard, note that it is also possible to formulate theories of the same phenomena through so-called product theories, which provide an accurate functional account of the phenomena but do not commit to a particular psychological mechanism or process (Vicente & Wang, 1998). Product theories may also be called blackbox theories or input-output theories. Product theories do not make predictions about processes (even though they may constrain processes). Thus, product theo- ries can be evaluated mainly by product measures. Process theories, in contrast, can be evaluated by using process measures when they are available and relevant (which are, relatively speaking, rare), such as eye movement and duration of pause in serial recall, or by using product measures, such as recall accuracy, recall speed, and so on. Evaluation of process theories using the latter type of measures can only be indirect, because process theories have to generate an output given an input based on the processes postulated by the theories (Vicente & Wang, 1998). Depending on the amount of process details specified, a computational model may lie somewhere along the continuum from pure product theories to pure process theories. There can be several different senses of “modeling” in this regard, as discussed in Sun and Ling (1998). The match of a model with human cognition may be, for exam- ple, qualitative (i.e., nonnumerical and rela- tive) or quantitative (i.e., numerical and ex- act). There may even be looser “matches” based on abstracting general ideas from ob- servations of human behaviors and then de- veloping them into computational models. Although different senses of modeling or matching human behaviors have been used, the overall goal remains the same, which is to understand cognition (human cogni- tion in particular) in a detailed (process- oriented) way. This approach of utilizing comp