The Reuben sandwich is a North American grilled sandwich composed of corned beef, Swiss cheese, sauerkraut, and Russian dressing or Thousand Island dressing, grilled between slices of rye bread. It is associated with kosher-style delicatessens but is not kosher, as it combines meat and cheese. As a Reuben combines both meat and dairy ingredients in the same meal, it is not kosher. However, it is frequently served at kosher-style restaurants. Kosher versions may be made using non-dairy imitation cheese, substitute vegetarian corned beef, or omitting the meat or the cheese.

Source: Wikipedia