Non-indigenous species (NIS) are organisms that enter an ecosystem beyond their native spatial range. The Sea Lamprey (Petromyzon marinus) was the first to enter the Great Lakes during the 1830s facilitated by the Erie and Welland canals. Since then, at least 185 other species have invaded the Great Lakes. Thirteen of these species have been labelled as invasive by causing ecological or economic harm. This book explores the data and tools that U.S. Great Lakes resource managers can use to more effectively prevent the establishment of aquatic NIS and maps the habitats of the Great Lakes most vulnerable to the entry of aquatic NIS while identifying particular NIS that have the potential to enter the U.S. Great Lakes ports.