What is sexy? As shown by this retrospective of Sports Illustrated Swimsuit Edition’s past fifty years of covers and models, the definition is always changing. With the 51st SI Swim issue set to land next month, we thought why not take a walk down memory lane and see the different models (and styles) from the past fifty years. Check out ten covers below featuring Kate Upton, Tyra Banks, Elle Macpherson, Christie Brinkley and more!