Part 01 | Origins to the end of the Original Five [1963 to 1975]

The beginning is pretty straightforward, but there are still a few issues where some X-Men appear outside their own comics that are worth noting, as well as continuity inserts (stories that were published later on but chronologically fit this era).

Vol. 00 - Origins
X-Men v2 #-1
X-Men #38-42 (back-ups) (Prof X. and Cyclops Origin)
X-Men #44-46 (back-ups) (Iceman Origin)
X-Men #54-56 (back-ups) (Angel Origin)
X-Men #49-53 (back-ups) (Beast Origin)

Vol. 01 - Children of the Atom
X-Men #1-2
Tales of Suspense #49 (1st story)
Avengers #3
X-Men #3
Professor Xavier and the X-Men (2nd half of the issue) (optional)
X-Men #4-5
Strange Tales #120 (1st story)
Fantastic Four #28
X-Men #6
Journey into Mystery #109 (1st story)
X-Men #7
Untold Tales of Spider-Man #21 (optional)
X-Men #8-13
Avengers 16 (p. 9-20) (Quicksilver & Scarlet Witch join the Avengers)
X-Men #14-18
X-Men: First Class #8
Fantastic Four Annual #3 (1965)
X-Men #19-23

Vol. 02 - Lonely are the Hunted
X-Men #24-31
X-Men Unlimited #42 (3rd story)
X-Men #32-39
Avengers #47-49 (Magneto pages only) (Magneto returns)
X-Men #40-45
Avengers #53
X-Men #46

Vol. 03 - The Sentinels Live
X-Men #47-48
Ka-Zar #2-3 (Angel stories)
Marvel Tales #30 (Angel story)
X-Men #49-66

Vol. 04 - Hulk: The Man Within (optional)
Savage Hulk v2 #1-4

Vol.05 - The Hidden Years
X-Men v2 #94 (2nd story)
X-Men: Hidden Years #1 (minus the Epilogue)
X-Men: Hidden Years #2-11
X-Men: Hidden Years #12 (minus the Epilogue)
X-Men: Hidden Years #13-19
X-Men: Hidden Years #12 (Epilogue)
Fantastic Four #102 (p. 5-23)
X-Men: Hidden Years #20
Fantastic Four #103 (p. 1-11)
X-Men: Hidden Years #21 (p. 1-18)
Fantastic Four #103 (p. 12-23)
Fantastic Four #104 (p. 1-12)
X-Men: Hidden Years #21 (p. 19-22)
X-Men: Hidden Years #22

Vol. 06 -  It’s Always Darkest Before The Dawn
[At this point the X-Men book went into reprints and the characters appear here and there in other comics.]

Amazing Spider-Man  #92
Amazing Adventures #9-10
Incredible Hulk #150
Amazing Adventures #11-13
Amazing Adventures #14 (p. 1-12)
Marvel Team-Up #4
Amazing Adventures #14 (p. 13-20)
Amazing Adventures #15-16
Incredible Hulk #161
X-Men Unlimited v2 #10 (Beast story) (optional)
Avengers #102 (p. 10-20)
Avengers #103-104
Avengers #110
Daredevil #99
Avengers #111
Incredible Hulk #172
Captain America #169-175
Incredible Hulk #180-181
X-Men Legends v2 #1-2
Marvel Team-Up #23
X-Men: First Class Finals #4 (last page) (optional)
Defenders #15-16
Giant-Size Fantastic Four #4

Continue to Part 02