A window screen (also known as insect screen, bug screen, fly screen, flywire, wire mesh, or window net) is designed to cover the opening of a window. It is usually a mesh made of metal, fibreglass, plastic wire, or other pieces of plastic and stretched in a frame of wood or metal. It serves to keep leaves, debris, bugs, birds, and other animals from entering a building or a screened structure such as a porch, without blocking fresh air-flow.

Most houses in Australia, the United States and Canada and other parts of the world have screens on windows to prevent entry of flying insects such as mosquitoes, flies and wasps. In some regions such as the northern United States and Canada, screens were required to be replaced by glass storm windows in the winter, but now combination storm and screen windows are available, which allow glass and screen panels to slide up and down.

For screens installed on aluminium frames, the material is cut slightly larger than the frame, then laid over it, and a flexible vinyl cord, called a spline, is pressed over the screen into a groove (spline channel) in the frame. The excess screen is then trimmed close to the spline with a sharp utility knife. Common spline sizes range from 3.6 mm (0.140 in) to 4.8 mm (0.190 in), in increments of 0.25 mm (0.010 in).

The spline is often manufactured with parallel ridges running along the length of the spline to provide a better grip and compression when it is pressed into the spline channel. A spline roller — a special tool that consists of a metal (or plastic) wheel on a handle — is used to press the spline into the frame. The wheel edge is concave, to help it hold the spline and not slip off to the side. Some spline rollers are double-ended and have both convex and concave rollers; the convex roller can be used to seat the spline deeper into the channel without risk of cutting the screen. Driving the spline into the channel tends to tension the screen on the frame, so the installer must avoid pre-tensioning the screen excessively to prevent the frame from becoming warped.