The passenger ocean liner is, without question, the greatest of all man-made construction, especially as exhibited at the peak of its development from the 1930s onwards. They were as large then as any skyscraper or any of the world’s greatest railway stations. But unlike these edifices, locked to the earth, static upon their foundations, the passenger ocean liners, fitted with machinery as large as that of any power station, were powerful, mobile monuments of man’s engineering prowess. Cleaving the waves at anything up to 30 knots, they were the leviathans and monarchs of the oceans, and truly were floating cities. In times of war the ocean liners were often requisitioned for use as troopships and thus played a key role in transporting large numbers of troops to and from the war zones.

In their heyday there was great rivalry between shipping lines, especially on the transatlantic route, as the liners were symbols of national prestige and you had to be the fastest and the best. It was the ocean liners and their scheduled liner routes linking Britain (and other ocean liner countries) with the diverse corners of the world that enabled people for the first time to travel long distances between continents on a regular basis.

Today the passenger ocean liner continues to evolve and adapt to new roles. Since the decline in their scheduled liner routes in the 1960s and 70s, they have become cruise liners and a holiday destination in their own right often becoming luxurious floating resorts with unparalleled amenities. Now in the early 21st century there is the emergence of the cruise liner as a "residential condominium ship" with the first example being The World launched in 2002. Thus cruise liners have evolved into floating communities with permanent on board "ocean residences". However some "residential condominium ships" also have on board suites for traditional cruise guests.