Hot tubs (or spas) have been used for longer than most people think. Romans had large public baths for hundreds of years, most of which were heated. Citizens from all walks of life enjoyed a good soak, and in those days, it was really more of a gathering place for meeting up and doing business. All this happened many centuries ago, before the therapeutic and hygienic properties of swirling hot water were discovered. Ruins of several of the Roman spa facilities are still accessible today. Modern Europeans have revived the Roman tradition of large spas.
Another culture with a long history of hot tub use is found in Japan. Called 'ofuro,' these smaller, stand-alone spas were used for centuries by Japanese families. There is evidence that Japanese people long ago understood (in a rudimentary way, at any rate) the importance of hygiene and something about the therapeutic value of taking a hot soak on a regular basis. Today, modern Japanese people continue to honor this old tradition. You will find many households that have elaborate hot tubs.
In America, the history of spas is a tale of two ages. Early in our history (1770s), stories were told about legendary heroes - George Washington and Alexander Hamilton, to name the two most prominent - soaking their bodies in hot springs to ease their pains and relax during those stressful times. Native Americans are thought to have discovered and used the springs for similar purposes, especially in upstate New York and the surrounding regions.
Later, in the early 1900s, a movement to introduce spa resorts sprang up in America, modeled after the European style that had been gaining in popularity for years. These resorts were built around naturally occurring hot springs, and even around man-made equivalents, in rare instances. They attracted mostly wealthy citizens who were lured by tales of their miraculous therapeutic effects on arthritis and other debilitating diseases.
The modern hot tub began springing up in America in the mid 1900s (1960s mostly). Following the Japanese ofuro design, small, personal wood spas were increasingly being used, mostly in California. Made from redwood or Mountain Cedar and circular (like a barrel cut in half), these hot tubs had a very basic design and often leaked. Nevertheless, the movement spread all across the nation. Today, you can find home hot tubs and spas made from wood or fiberglass-reinforced plastic all over the US.