As one of the first books in child second language acquisition (SLA), this book focuses on the core area of tense-aspect morphology, reporting on three L1-Italian children learning L2 English vs. three L1-English children learning L2 Italian. An innovative longitudinal/bidirectional research design, where two languages represent both source and target, show effects of language transfer in learners that, because of their age, still have potential to become native-speakers of the target. An unusual feature of this book is that relevant studies of acquisition of L2 Italian, some heretofore only in Italian, are reviewed, incorporated into the study and made available to a more general audience. Though the main focus is on child SLA, crucial comparisons to both first language acquisition vs. adult SLA are presented. This approach will thus be of interest more generally to readers in first and second language acquisition and child development.