Window and door installation quality significantly affects building performance and durability. Improper installation allows water intrusion and air leakage. Modern windows are insulated units (IGUs) with thermal breaks and low-emissivity coatings. Installation begins with preparation of rough openings. Window openings must be properly sized per manufacturer specifications; too small creates installation stress, too large requires excessive shimming. Rough opening dimensions must be square, level, and plumb. Window frames are set in the opening with shims supporting proper height and level. Fastening is typically with nails through the frame flange. Fastener spacing per manufacturer specification ensures adequate support. Perimeter gaps are sealed with insulation and caulk preventing air and water leakage. Flashing above windows directs water away from the building. Head flashing must extend above the top of the window and slope outward. Side and sill flashing direct water to the exterior. Caulking of all seams creates a weather-tight seal. Compatible caulk that can flex with building movement lasts longer than rigid caulk. Interior drywall and exterior cladding are sealed to the window frame. Interior seal tapes and exterior sealant create continuous air and water barriers. Installation quality determines whether windows perform as designed. Improper installation can cause failures even with quality windows. Training of installation crews on proper procedures ensures quality results. Manufacturer requirements for flashing and sealant must be followed to maintain warranties.