A pastry bag is used to pipe semi-solid foods by pressing them through a narrow opening at one end, for many purposes including cake decoration. It is filled through a wider opening at the opposite end, rolled or twisted closed, and then squeezed to extrude its contents.

Though a circular nozzle is quite useful for making round shapes and for filling pastries such as profiteroles, many differently-shaped nozzles are commonly

used to produce star, leaf, and flower-petal shapes.

Aside from icing, pastry bags are commonly used to shape meringue and whipped cream, and to fill doughnuts with jelly or custard. When presentation is especially important, fluted tips can be used to shape savory foods such as filling for devilled eggs, whipped butter, and mashed potatoes (especially for Pommes chesse).

A high-quality bag is often made from nylon, polyester, or waterproofed cotton, with a collection of interchangeable chrome-plated or stainless steel tips. Each tip is cone-shaped, with a base too large to fit through the small opening in the bag; they are to be inserted through the larger opening before food is spooned in. Less expensive models are generally of plastic film with screw-on plastic tips (see image, above), while many foods

(including frosting and pressurized "spray can" whipped cream) can be purchased in disposable packaging designed to serve the function of a pastry bag.

A simple pastry bag can be made by rolling cooking parchment or wax paper into a cone, filling it, folding the wide end several times to close it, and then cutting the tip into whatever shape is desired. This is especially useful for small quantities of melted chocolate, since a very small hole can be cut and the bag can be discarded when it cools and becomes clogged.