The goal of this study was to examine sexual harassment in early adolescence. Available data indicate that peer to peer sexual harassment is prevalent in high school and is associated with psychosocial problems for both victims and perpetrators. For the present study, we adopted a developmental contextual model to examine the possibility that this behavior develops during the late elementary and middle school years and is linked to the biological and social changes that occur at this time. Youths from Grades 6–8 (N = 1,213) enrolled in seven elementary and middle schools in a large south-central Canadian city were asked to report on their sexual harassment behaviors with same- and cross-gender peers; their pubertal development, and the gender composition of their peer network. The results revealed that cross-gender harassment was distinct from same-gender harassment, increased in frequency from Grade 6 to Grade 8, and was linked to pubertal maturation and participation in mixed-gender peer groups. The implications of a developmental contextual model for understanding the emergence of this problematic behavior in adolescence are discussed.