This symposium issue is first and foremost about crossing boundaries. The people readers have met in these pages—enslavers and enslaved, traders and purchasers, abolitionists and insurrectionaries—were mobile, and their mobility had consequences. The slave traders who changed flags as they moved across international waters are only the most visible exemplars of this phenomenon. Crossing geographic borders often meant crossing boundaries of race and status as well. All of these articles in one form or another address the question of what it means to cross lines: between “slave” and “free,” “legal” and “illegal,” “past” and “present.”