Measures of the amounts of time infants spent asleep, awake-content, feeding, fussing, and crying at 2, 6, 12, and 40 weeks of age were examined using multilevel analysis. This method enables the proportion of the variance in each behavior due to individual differences to be compared to the proportion due to age changes (development) and to day-to-day fluctuations at each age in the same infants. Day-to-day fluctuations were found to account for the largest proportion of the variance in amounts of sleeping, fussing, and crying (between 44% and 53%), testifying to the importance of instability in these behaviors as a characteristic of infancy. Against this background, both development and individual differences explained substantial proportions of the variance, with a somewhat different picture in each area of behavior. Amounts of waking and feeding were mainly accounted for by development, and no evidence of enduring individual differences was found. For sleeping, development and individual difference each contributed approximately a quarter of the variance, and the amounts infants slept remained moderately stable from 6 weeks to 9 months of age. Crying decreased linearly with age, with development accounting for 38% and individual difference 15% of the variance. Fussing proved a more stable characteristic than crying, and "high fussers" at 6 weeks of age were particularly likely to retain this characteristic at 9 months, whereas amount of crying in the first 3 months did not predict 9-month behavior. The study's clinical, conceptual, and methodological implications are discussed.