A defense line or fortification line is a geographically recognizable line of troops and armament, fortified and set up to protect a high-value location or defend territory.

A defense line may be based on natural difficult terrain features, such as rivers or marshes, mountain ranges, or coastlines; temporary field fortification works such as trenches; and/or more permanent fortifications such as fortresses and bunkers.

Some notable defense lines include:

• Great Wall of China, China
• Sassanian defense lines
  • Great Wall of Gorgan, Persia
  • Derbent Walls
• Defense lines of the Netherlands
• Median Wall (before 401 BC)
• Limes Germanicus, Germany
• Hadrian's Wall, United Kingdom (122)
• Antonine Wall, United Kingdom (142)
• Offa's Dyke, Wales (early 5th century)
• The Pale, Ireland (late 12th century)
• Serpent's Wall, Ukraine
• The Zasechnaya cherta [ru] (Russian: Засечная черта)) on the southern and eastern borders of the Tsardom of Russia
• Western Russian fortresses, Russia
• Civil War Defenses of Washington, United States
• Victoria Lines, Malta (1875)
• Hindenburg Line, France (1916)
• Maginot Line, France (1929)
• Schuster Line, Luxembourg
• Metaxas Line, Greece (1936-1941)
• Siegfried Line, Germany
• Mannerheim Line, Finland (1939–1940)
• K-W Line, Belgium (1939)
• Salpa Line, Finland (1940)
• Gothic Line, Italy
• Winter Line (Gustav, Bernhardt and Hitler Lines), Italy (1943-1944)
• Panther–Wotan line, Russia (1943)
• Pusan Perimeter, South Korea (1950)
• McNamara Line, South Vietnam (1966)
• Bar Lev Line, Sinai Peninsula (1973)

• Russia–Ukraine barrier, Ukraine
• Wagner Line, Ukraine
• Muhamalai FDL, Sri Lanka
• Nagarcoil FDL, Sri Lanka
• Baltic Defence Line, Baltic states