Purpose This small, prospective cohort study of 162 12- to 15-year-old girls examines timing of changes in sexual cognitions and behaviors (breast fondling, genital contact, and sexual intercourse) over a 1-year period. Methods Girls from community sites in New York City provided information about sexual experiences and related cognitions (arousability, sexual agency, abstinence attitudes, perceived parental and peer approval, and sexual self-esteem) in 2 interviews 1 year apart. Results Percentages who reported breast fondling, genital contact, and sexual intercourse (18%, 24%, and 6%) increased significantly over the year (42%, 44%, and 19%) and with age. For each behavior, analyses compared girls who did not report the behavior at either Time 1 or 2, those who reported by Time 2 but not 1, or those who reported at both time points. Girls with no breast fondling experience at either time point had stronger abstinence values, and lower arousability, agency, peer approval, and sexual self-esteem scores compared to girls who initiated breast fondling over the year (transitioners). Transitioners were markedly similar in sexual cognitions to girls with this experience before Time 1, suggesting that changes in sexual cognitions precede actual experience. A similar pattern was found between groups for genital contact. Few differences between groups by comparison were noted for sexual intercourse, which occurs later in the trajectory. Conclusions Sexual experiences that occur before intercourse (e.g., breast fondling) are central to research on sexual development and related to greater changes in girls’ sexual cognitions than is intercourse. Changes in cognitions precede, rather than follow, new sexual experiences.