X-Men vol 1 #1  (1st X-Men, Magneto; Lee & Kirby begin)
X-Men vol 1 #2  (1st Vanisher)
X-Men vol 1 #3  (1st Blob)
X-Men vol 1 #4  (1st Brotherhood of Evil Mutants)
X-Men vol 1 #5

X-Men vol 1 #6
X-Men vol 1 #7
X-Men vol 1 #8  (1st Unus)
X-Men vol 1 #9 
X-Men vol 1 #10  (1st Ka-Zar) 

X-Men vol 1 #11  (1st Stranger)
X-Men vol 1 #12  (1st Juggernaut; origin of Professor X)
X-Men vol 1 #13
X-Men vol 1 #14  (1st Sentinels)
X-Men vol 1 #15  (origin of the Beast)

X-Men vol 1 #16
X-Men vol 1 #17  (Kirby's final issue)
X-Men vol 1 #18  (full Werner Roth art begins)
X-Men vol 1 #19  (Lee's final issue; 1st Mimic)
X-Men vol 1 #20  (Roy Thomas scripts begin)

X-Men vol 1 #21
X-Men vol 1 #22
X-Men vol 1 #23
X-Men vol 1 #24
X-Men vol 1 #25

X-Men vol 1 #26
X-Men vol 1 #27  (Mimic joins)
X-Men vol 1 #28  (1st Banshee)
X-Men vol 1 #29  (Mimic leaves)
X-Men vol 1 #30

X-Men vol 1 #31  (1st Cobalt Man; 1st Candy Southern)
X-Men vol 1 #32
X-Men vol 1 #33
X-Men vol 1 #34
X-Men vol 1 #35  (Roth's final issue; 1st Changeling)

X-Men vol 1 #36
X-Men vol 1 #37
X-Men vol 1 #38  ("Origins of the X-Men" begins)
X-Men vol 1 #39  (1st individual costumes)
X-Men vol 1 #40

X-Men vol 1 #41
X-Men vol 1 #42  (Professor X "dies")
X-Men vol 1 #43
X-Men vol 1 #44  (Final Thomas issue; 1st Gary Friedrich script)
X-Men vol 1 #45  (Avengers crossover)

X-Men vol 1 #46  (1st Arnold Drake script)
X-Men vol 1 #47 
X-Men vol 1 #48
X-Men vol 1 #49  (1st Polaris; 1st Mesmero)
X-Men vol 1 #50 

X-Men vol 1 #51
X-Men vol 1 #52
X-Men vol 1 #53
X-Men vol 1 #54  (1st Havok; final Drake script)
X-Men vol 1 #55

X-Men vol 1 #56  (Thomas/Adams run begins)
X-Men vol 1 #57
X-Men vol 1 #58  (1st Havok in costume)
X-Men vol 1 #59  (1st Sauron)
X-Men vol 1 #60

X-Men vol 1 #61
X-Men vol 1 #62  (1st Savage Land Mutates)
X-Men vol 1 #63
X-Men vol 1 #64  (1st Sunfire)
X-Men vol 1 #65  (Professor X returns)
X-Men vol 1 #66  (Final Silver Age issue)