Rankine's Theory is a method used in civil engineering to analyze the stability of retaining walls and slopes. It provides a way to calculate the lateral earth pressure exerted by soil on a structure, considering factors like soil weight and wall friction. The theory assumes that the soil behaves like a fluid and that the wall is rigid. The theory is based on the principles of equilibrium and uses the concept of active and passive earth pressure. William John Macquorn Rankine, a 19th-century engineer, developed this theory, which remains fundamental in geotechnical engineering today.