A hot tub is a large home-made or manufactured tub or small pool full of heated water and used for soaking, relaxation, massage, or hydrotherapy. In most cases, they have jets for massage purposes. Hot tubs are usually located outdoors, and are often sheltered for protection from the elements, as well as for privacy.

There are two different styles of hot tubs:

• Simple wooden-staved soaking tubs
• One piece plastic tubs (usually referred to as spas)

Hot tubs are usually heated using an electric or natural gas heater, though there are also submersible wood-fired heaters, as well as solar hot water systems. Hot tubs are also found at natural hot springs; in this case, the water may be dangerously hot and must be combined with cool water for a safe soaking temperature.

Water sanitization is very important in hot tubs, as many organisms thrive in a warm, wet environment. Maintaining the hot tub water chemistry is also necessary for proper sanitization and to prevent damage to the hot tub.

Of the 130-odd manufacturers in North America in 2005, the top twenty each produced more than 7,000 spas annually. Fact: date=February 2007 China is quickly becoming the largest supplier of Hot tubs. Fact: date=November 2007

Wooden tubs

This style of hot tub is constructed with wooden staves and steel bands, very much like a very large barrel. The staves are usually made of redwood, cedar, or teak, with most older tubs made of redwood. Wooden hot tubs are often quite deep, 90-120cm (36"-48"), and can be inset within a wooden deck for ease of entry. Inside the tub, wooden bench seating is common, forming a ring around the inner circumference of the tub.

One Piece Hot Tubs

One piece spas are usually shallower than wooden tubs, usually being 80-95cm (32" to 36") in height to fit through doors and narrow hallways. Cranes are occasionally used to place one piece spas in a backyard or other location that does not have adequate clearance for carrying the tub.

Spas usually have between one and four water pumps, with one circulation pump serving the heating and filtration water loop and the other(s) driving the hydrotherapy jets. Sophisticated computer controls are now common and many tubs now are equipped with extensive lighting, sound systems, and even flat panel televisions with DVD players.

Construction of a one-piece spa

The spa shell is the exterior of the tub, and is composed of a surface and an understructure that are bonded together during the manufacturing process. The surface is the source of the color, look and feel of the spa, so it should resist deterioration due to the sun, spa chemicals, or normal wear and tear. Some high-end shells have special coatings to make them more stain resistant or have anti-bacterial ingredients molded into the shell material.