Ship canals distort space and time. Some of them create routes across landmasses that would otherwise have to be circumnavigated, dramatically shortening the maritime distance between two points. Others turn landlocked towns into seaports, bringing ocean-going vessels far inland and challenging the interests of older ports. Ship canals are therefore a disruptive force in their own right, and they have historically served as enablers of others, not least the steamship. Canals offer important lessons in how states, navies, entrepreneurs, port authorities and engineers have visualised and manipulated the relationship between land and sea, often on a monumental scale, and also in how those institutions and individuals have adapted to the consequences of such actions. Most major ship canals were a product of a particular time, and need to be interpreted as part of European and American economic and imperial expansion from the second half of the 19th century. Their role changed, but was still important, in the 20th century. Even with the development of aviation, the sea remained fundamental to the security and prosperity of states. Warships did not lose their strategic importance in projecting state influence overseas, while cargo shipping has always been the cheapest means of carrying most goods. Today, the shipping industry defines classes of vessels according to whether they can navigate the world's major canals, and the demands of ever-larger ships drive periodic reassessments of the operation and capacity of the canals themselves.