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A balloon is an inflatable flexible bag filled with a gas, such as helium, hydrogen, nitrous oxide, oxygen, or air. Modern balloons can be made from materials such as rubber, latex, polychloroprene, or a nylon fabric, while some early balloons were made of dried animal bladders, such as the pig bladder. Some balloons are used for decorative purposes, while others are used for practical purposes such as meteorology, medical treatment, military defense, or transportation. A balloon's properties, including its low density and low cost, have led to a wide range of applications. The inventor of the rubber balloon, (the most common balloon) was Michael Faraday in 1824, via experiments with various gases.

Party balloons are mostly made of a natural latex tapped from rubber trees, and can be filled with air, helium, water, or any other suitable liquid or gas. The rubber's elasticity makes the volume adjustable.

Filling the balloon with air can be done with the mouth, a manual or electric inflater (such as a hand pump), or with a source of compressed gas.

When rubber or plastic balloons are filled with helium so that they float, they typically retain their buoyancy for only a day or so, sometimes longer. The enclosed helium atoms escape through small pores in the latex which are larger than the helium atoms. Balloons filled with air usually hold their size and shape much longer, sometimes for up to a week.

Even a perfect rubber balloon eventually loses gas to the outside. The process by which a substance or solute migrates from a region of high concentration, through a barrier or membrane, to a region of lower concentration is called diffusion. The inside of balloons can be treated with a special gel (for instance, the polymer solution sold under the ‘Hi Float’ brand) which coats the inside of the balloon to reduce the helium leakage, thus increasing float time to a week or longer.

Beginning in the late 1970s, some more expensive (and longer-lasting) foil balloons made of thin, unstretchable, less permeable metallised films such as Mylar (BoPET) started being produced. These balloons have attractive shiny reflective surfaces and are often printed with color pictures and patterns for gifts and parties. The most important attribute of metallised nylon for balloons is its light weight, increasing buoyancy and its ability to keep the helium gas from escaping for several weeks. Foil balloons have been criticized for interfering with power lines.

Professional balloon party decorators use electronic equipment to set the exact amount of helium to fill the balloon. For non-floating balloons, air inflaters are used. Professional quality balloons are used, which differ from most retail packet balloons by being larger in size and made from 100% biodegradable latex.