The 1973 NCAA Division I football season was the first for the NCAA's current three-division structure. Effective with the 1973–74 academic year, schools formerly in the NCAA "University Division" were classified as Division I (later subdivided for football only into today's Division I FBS and Division I FCS). Schools in the former "College Division" were classified into Division II, which allowed fewer athletic scholarships than Division I, and Division III, in which athletic scholarships were prohibited.
In its inaugural season, Division I had two NCAA-recognized national champions, and they faced each other at year's end in the Sugar Bowl. The New Orleans game matched two unbeaten teams, the Alabama Crimson Tide (11-0), ranked #1 by AP and UPI, and the Notre Dame Fighting Irish (10–0), ranked #3 by AP and #4 by UPI.
While both wire services ranked Alabama #1 at the end of the regular season, AP took another poll after the bowl games. By agreement with the American Football Coaches' Association, however, UPI bestowed its championship before the postseason bowl games. Thus, Alabama was crowned champion by UPI on December 4, 1973. UPI ranked Notre Dame #4. One coach had given the Irish a first place vote, compared to 21 for Alabama.
In a game where the lead changed six times, Notre Dame won by a single point, 24–23, to claim the AP national championship. During the 20th Century, the NCAA had no playoff for the college football teams that would later be described as "Division I-A". The NCAA Football Guide, however, did note an "unofficial national champion" based on the top ranked teams in the "wire service" (AP and UPI) polls. The "writers' poll" by Associated Press (AP) was the most popular, followed by the "coaches' poll" by United Press International) (UPI). In 1973, the UPI issued its final poll before the bowls, but the AP Trophy was withheld until the postseason was completed. The AP poll in 1973 consisted of the votes of as many as 63 sportswriters and broadcasters, though not all of them voted in every poll. UPI's voting was made by 34 coaches. Those who cast votes would give their opinion of the ten best teams. Under a point system of 20 points for first place, 19 for second, etc., the "overall" ranking was determined.
October 6: #1 Ohio State beat Washington State 27-3. #2 Nebraska won at Minnesota 48-7. #3 Alabama beat Georgia at home, 28-14. #4 USC won at Oregon State, 21-7. #5 Michigan beat Oregon 24-0. The poll remained unchanged: 1.Ohio State 2.Nebraska 3.Alabama 4.USC 5. Michigan
#5 Michigan won at Michigan State, 31-0. #6 Oklahoma beat #13 Texas 52-13 in Dallas #7 Penn State beat visiting Army, 54-3, to extend its record to 5-0-0 and rise to the top five. The poll: 1.Ohio State 2.Alabama 3.Oklahoma 4.Michigan 5.Penn State
November 10: #1 Ohio State recorded its 3rd shutout, a 35-0 win over visiting Michigan State. #2 Alabama was idle. #3 Oklahoma won at #10 Missouri 31-3. #4 Michigan beat Illinois 21-6. #5 Notre Dame won at #20 Pittsburgh 31-10. The poll: 1.Ohio State 2.Alabama 3.Oklahoma 4.Michigan 5.Notre Dame
1.Ohio State 2.Alabama 3.Oklahoma 4.Michigan 5.Notre Dame
On Thanksgiving Day, #2 Alabama beat #7 LSU 21-7 and #5 Notre Dame beat Air Force 48-15. The next day, #3 Oklahoma beat #10 Nebraska 27-0. The big matchup was on Saturday, November 24, in Ann Arbor, Michigan, where #1 Ohio State (9-0-0) and #4 Michigan (10-0-0) met. The two teams played to a 10-10 tie. Alabama, still unbeaten and untied, took over the top spot in the next poll: 1.Alabama 2.Oklahoma 3.Ohio State 4.Michigan 5.Notre Dame
In the final regular season poll, the top six schools were unbeaten. 1.Alabama (11-0) 2. 2.Notre Dame (10-0) 3.Oklahoma (10-0-1) 4.Ohio State (9-0-1) 5.Michigan (10-0-1) and 6.Penn State (11-0). The other major college unbeaten, Miami (Ohio) (10-0-0), was #15. Oklahoma, however, was on probation for having used an ineligible player in three 1972 games, and was ineligible to play in a bowl game. #1 Alabama and #2 Notre Dame accepted invitations to play in the Sugar Bowl.
Each team may use its own legal ball when it is in possession.
Free substitution is allowed, however substitutes both checking in and leaving the field of play must do so through their own team areas, and are not required to stay in or leave for one play. This change would be rescinded in the 1974 season.
The fair catch signal is standardized as waving one arm side to side before the ball is caught. Any other signal (including shielding of the eyes) is considered invalid, and players making a fair catch with either a valid or invalid signal are protected from being tackled or blocked. Previously, receivers making an invalid signal were not afforded any protection.
All players are required to wear a mouth guard and a fully buckled chin strap. Violators must leave the field until the violation is corrected.
If an illegal forward pass is completed in the end zone (whether to an eligible or ineligible receiver), the ball is dead and the penalty enforced.
Alabama and Notre Dame had never met in a college football game before their encounter in the Sugar Bowl, which was played on December 31, 1973. Two legendary coaches, Bear Bryant and Ara Parseghian brought their teams to New Orleans, and the game was a thriller. The Irish scored first, but missed the extra point. After Alabama took a 7–6 lead, freshman Al Hunter returned the ensuing kickoff 93 yards for a touchdown, and a two-point conversion put Notre Dame up 14–7. Alabama went ahead 17–14 in the third, but a fumble on their own 12-yard line gave the Irish a chance to make it 21–17. In the fourth quarter, Bama got back the lead on a trick play, as quarterback Richard Todd handed off to running back, Mike Stock, who then fired a touchdown pass back to Todd; but Bill Davis, who had made 51 of 53 extra point attempts in his career, was wide right, and the score stayed 23–21. In the final minutes, Notre Dame's Bob Thomas (who had missed the earlier point after try) kicked a 19-yard field goal that gave the team the 24-23 win. Asked whether Notre Dame would be voted #1, Coach Parseghian replied, "Certainly. What was the final score?" 
The final AP writers' poll was split. Notre Dame received a majority of the first place votes, 33 out of 60, followed by #2 Ohio State (11 votes) and #3 Oklahoma (16 votes, but fewer points overall). The #4 spot (held by Notre Dame in the final UPI poll) went to Alabama. UPI, who crowned Alabama as national champion at the end of the regular season, would begin holding the coaches' poll after the bowl games beginning with the 1974 season.
John Cappelletti had the third best year in Penn State history when he gained 1,117 yards rushing in 1972. In 1973, he had the second best year in Penn State history rushing for 1,522 yards. In his two-year running career, he gained 100 yards in the thirteen games and had a career total of 2,639 yards and twenty-nine touchdowns for an average of 120 yards per game and 5.1 yards per carry. John's acceptance speech at the Heisman Dinner (with Vice President Gerald Ford next to him on the dais) was considered the most moving ever given at these ceremonies, as he honored his brother Joey, a victim of leukemia.