A gas cylinder is a pressure vessel for storing gases at higher than atmospheric pressure. Its contents may be in the form of compressed gas, vapor over liquid, supercritical fluid, or dissolved in a solid or gaseous substrate material. The container has a pressure-operated valve to control the release of its contents. A cap, collar or neck ring protects the valve assembly when not in use. A valve assembly is usually connected to a regulator when the cylinder is being used to supply its contents to a receiving apparatus such as a torch or other equipment.

The handling of gas cylinders poses both chemical and physical hazards. The pressurized contents can be toxic, flammable, oxidizing, corrosive, or inert, and any leak from the cylinder, its valve, or the system used to deliver the gas to a tool or piece of machinery is dangerous. There is also a physical danger from the weight of the cylinder, which can become a projectile if it falls over or is knocked over.

Typical gases stored in gas cylinders are oxygen, nitrogen, carbon dioxide, acetylene, and liquefied petroleum gas (LPG). Because these are often dispensed at high temperatures, the cylinders are constructed of materials that can withstand these conditions. These cylinders are also often subject to additional pressure from the pressure-operated valve assembly and the ambient temperature. This additional pressure can cause the cylinder to fail mechanically, potentially resulting in injury or death.

Accidents involving gas cylinders are relatively common, but many of these can be prevented with simple precautions. These include properly securing the cylinder to a cart, restricting access to the cylinder by children, and ensuring that the valve is never open except when the cylinder is being filled or used to supply a tool or machine. It is also important to ensure that a gas cylinder is never placed in water, and that the cap is kept on when not in use.

If a gas cylinder is dropped, the pressure within it can be released quickly and with great force. This can rupture the cylinder and its contents, and can also break any other chemical containers or equipment in its path. It is also important to keep a gas cylinder from locations where it could be damaged by heat sources, and to use a proper cylinder cart when transporting it.

Cylinders must be labeled to indicate the contents and supplier. This is particularly important for cylinders that are a combination of different gases. For example, a cylinder that contains both acetylene and hydrogen must be constructed to inhibit the decomposition reaction of acetylene and prevent the occurrence of an explosive decomposition event. Additionally, cylinders must be examined on a regular basis to determine if they are still safe for continued use. This is governed by the requirements of the Carriage Regulations for new cylinders, and by Part 4 of the Carriage Regulations for old cylinders. Modifications and major repairs to these cylinders are not permitted, unless authorized by the appropriate inspection body.