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A window screen, insect screen, or fly screen is a metal wire, fiberglass, or other synthetic fiber mesh, stretched in a frame of wood or metal, designed to cover the opening of an open window. Its primary purpose is to keep insects, leaves, birds, and animals from entering a building or a screened structure such as a porch, while permitting fresh air-flow. Most houses in North America have screens on all operable windows, which are most useful in areas that have large mosquito populations. Screens in North America were traditionally replaced with glass "storm windows" in cold climates to insulate the window during the winter, but frames combining both storm and screen panels have become the most common type of screen currently used in cold climates.

For screens installed using aluminum frames, the window screen is typically cut larger then the frame, laid over it and a rubber cord (called a spline) is pressed over the screen into a special groove (called a spline channel) in the frame, the screen is then trimmed. A special tool that looks like a wheel on a handle (called a spline roller) is used to press the cord into the frame. The wheel has an indentation in its edge to help it catch the cord and not slip off. If the wheel is not available a good substitute is a medium thickness wire clothes hanger. Try to find a hanger with a wire thickness just a bit smaller then the cord, and use a rounded corner to press the rubber back into its groove. When installed using wooden frames, the screen fabric is tacked or stapled onto the frame. A small wooden molding is then nailed over the ragged edge. The screening fabric needs to be stretched tightly before nailing, but not so tightly as to deform the fabric. Because of corrosion problems, metal screening fabrics other than aluminum should not be used in aluminum frames.

The most common materials used for insect screening material are aluminum and fiberglass. Aluminum is generally available in natural aluminum or in an applied charcoal color. The charcoal is much less visible and should be preferred where the view through the screens as well as the external appearance of the windows are important considerations. Fiberglass is available in light gray as well as charcoal colors, the charcoal again offering better viewing and appearance. Fiberglass is less expensive, and has the advantage of not "denting" when hit or pushed. However, the fiberglass mesh is somewhat more opaque than aluminum mesh, which darkens the external appearance of the window and reduces the amount of light transmitted from outside. For applications requiring greater strength, such as screened doors, nylon, and polyester screening is also available.

A new screening material that claims to be virtually transparent is currently manufactured by W. L. Gore & Associates, marketed by Pella Windows under the name “Vivid View”®.

The premier material for insect screening is bronze. Bronze will give much longer service than either aluminum or fiberglass. When first installed, it has an unattractive gold color which weathers to an unobtrusive dark charcoal within a year or less. Bronze is somewhat more resistant to denting than aluminum. The very high cost of bronze screening explains why it is not more commonly used. Less common screen fabrics include copper, brass, stainless steel and galvanized steel.

In addition to insect screening, denser screen types that also reduce sunlight and heat gain are available. These offer significant potential energy savings in hot climates.

Several manufacturers offer screens that roll into a pocket when not in use. These are available for casement windows as well as other types of window and door openings.