Despite its salience in terms of both prevalence and distress occasioned in parents, the nature and causes of infantile colic remain poorly understood. Causes in the gut (abnormal sensitivity to dietary components, excessive gas, intestinal hypermotility, hormonal factors) and alternative explanations (variant of normal crying behavior, effect of atypical parenting, manifestation of problems in parent-infant interaction) are critically reviewed. Gut pathology may, at best, explain a minority of cases. Although evidence in support of both sets of causes exists, it is frequently undermined by methodologic limitations, notably inadequate definition of the subjects studied. Conceptual issues have also constrained progress in resolving some of the dilemmas, specifically the assumption that gut and non-gut causes are mutually exclusive, and that presence of symptoms necessarily implies abnormalities of function. It is proposed that (1) colic may not be a homogeneous group, but a loose description for the crying of various clinical and etiological subgroups; and (2) that most cases may be conceptualized as resulting from behavioral and biologic interactions in which both the behavioral and biologic factors are functioning normally rather than pathologically.