The following is a list of all of the times Northwestern has defeated a top-ten ranked program.  Note that, with a few exceptions, these rankings are based on the Associated Press poll, which began in 1936. 

There are some wins before 1936 that should be mentioned.  Even though there were no rankings, it is likely that the following teams would have been ranked in the national top ten when they fell to NU:

• 1895: NU 22, Chicago 6.  Home Game.  Chicago was 6-0 coming into the game, and would likely have bucked east-coast bias at the time for a top-ten ranking.
• 1896: NU 46, Chicago 6.  Road Game.  This was a BIG one.  Chicago was 12-0 coming into the game, and had outscored its opponents 325 to nothing up to hitting the Purple buzzsaw.  Chicago would have been likely ranked either 1 or 2 coming into the game.
  
• 1917: NU 21, Michigan 12.  Home Game.  Michigan had scorched its first eight opponents, but had suffered its first loss, to Penn, just a week before losing to NU.  It is possible that the Wolverines would still have been ranked in a top ten at the time.
• 1925: NU 3, Michigan 2.  Home Game (played at Soldier Field).  Another big one.  Michigan was undefeated and unscored upon, having won five games by a total of 180 to 0.  Michigan would have been ranked number one, without a doubt.
• 1927: NU 19, Ohio State 13.  Road Game.  Ohio State was highly regarded coming into the season and was 2-0 coming into the game.  It is likely that OSU would have cracked a top ten list.
• 1930: NU 19, Ohio State 2.  Home Game.  Same story as 1927-- OSU was 2-0 coming in, and very highly regarded.
• 1930: NU 14, Tulane 0.  Home Game.  Tulane was 1-0 coming into Dyche Stadium and had not lost a game since early 1928-- 15 straight games!  Tulane would have been in the top five.
• 1935: NU 14, Notre Dame 7.  Road Game.  Notre Dame would almost certainly have been a number one team if the polls had started in '35, rather than '36.  The Irish were 6-0 and had just knocked off Ohio State in the original "Game of the Century."

...And here are the NU wins against top ten teams since the AP poll's debut:

• 1936: NU 6-0 over #1 Minnesota.  Home Game.  The AP Poll had begun just two weeks before, and Minny was ranked #1 each week.  Until NU upended them at Dyche Stadium.  The only time NU has defeated an AP #1 team.   NU briefly took the #1 crown, until Minny reclaimed it.
• 1938: NU 6-3 over #2 Minnesota.  Home Game.
• 1942: NU 3-0 over #7 Texas.  Home Game.  NU's only win in '42, but it was a huge one.
• 1945: NU 26-14 over #4 Purdue.  Home Game.
• 1948: NU 19-16 over #8 Minnesota.  Home Game.
• 1948: NU 20-14 over #4 Cal.  1949 Rose Bowl.  Highest ranked opponent NU had beaten away from Evanston, until 2009.
• 1949: NU 21-20 over #7 Michigan.  Home Game.
• 1950: NU 14-7 over #6 Illinois.  Home Game.
• 1958: NU 29-28 over #10 Washington State.  Home Game
• 1959: NU 45-13 over #2 Oklahoma.  Home Game.  Probably the most famous game involving NU knocking off a top four team.  Parseghian's signature win.
• 1959: NU 14-10 over #5 Iowa.  Road Game.  Just a week after the win over Oklahoma, NU stormed into Iowa and did this.
• 1961: NU 12-10 over #8 Notre Dame.  Road Game.  The third of four straight wins vs. the Irish.
• 1962: NU 18-14 over #6 Ohio State.  Road Game.
• 1967: NU 12-7 over UPI #1 Miami.  Home Game.  Miami was #8 in the AP.
• 1995: NU 17-15 over #9 Notre Dame.  Road Game. 
  
• 1995: NU 19-13 over #7 Michigan.  Road Game.
• 1996: NU 17-16 over #6 Michigan.  Home Game.
• 2000: NU 47-44 over #7 Wisconsin.  Road Game. 
  
• 2004: NU 33-27 over #7 Ohio State.  Home Game.
• 2009: NU 17-10 over BCS #4 Iowa.  Road Game.  Iowa was #8 in the AP.