The literature on "infantile colic" or "paroxysmal fussing" is reviewed. The frequent occurrence of a regular evening fussy period in two groups of infants is reported as indicative that this is a normal physiological event of the first few weeks of life. The longitudinal records of 98 infants who were study subjects of the Yale Rooming-In Project were analyzed with respect to incidence, duration, and severity of "paroxysmal fussing." Forty-eight of the infants were classified as "fussy" and 50 as "contented." The "fussy" babies typically began to have their paroxysms in the second week of life and ceased to have them at about eight weeks. Almost all of them were more fussy in the evening hours than in the morning. The rationale of various methods of treatment is discussed. The "fussy" group was similar to the "contented" group as regards details of feeding, birth weight, weight gain, sex, educational level of the mother, and family history of allergy. Of the 48 "fussy" infants, family tension was judged to be an important contributing cause in 22 cases, allergy in six cases; both allergy and family tension together in nine cases; in 11 cases there was no apparent cause. "Paroxysmal fussing" or "infantile colic" is possibly one of the earliest somatic responses to the presence of tension in the environment. The particular degree to which any infant reacts is probably determined by constitutional factors.