It's Adventure Comics' 500th anniversary issue! One hundred and fifty pages of excitement!
...well, it's one hundred and fifty pages of excitement for die-hard Legion fans, for someone like me, it was one hundred and forty-seven pages of kinda dull material, and three pages of a cool retrospective of the history of Adventure Comics(which you can read here).
Behind the fine cover by Keith Giffen and Larry Mahlstedt are these Silver Age Legion adventures:
"The Secret of the Mystery Legionnaire" by Jerry Siegel and John Forte
"The Legion of Substitute Heroes" by Edmond Hamilton and Forte
"The Secret Power of the Mystery Super-Hero" by Hamilton and Forte
"The Return of Lightning Lad" by Hamilton and Forte
"The Legion of Super-Monsters" by Hamilton and Forte
"The Doom of the Super-Heroes" by Hamilton and Forte
"The War Between the Substitute Heroes and The Legionnaires" by Hamilton and Forte
"The Super-Sacrifice of the Legionnaires" by Hamilton and Forte
"The World of Doomed Olsens!" by Siegel, Curt Swan, and George Klein
"The Condemned Legionnaires" by Hamilton, Swan, and Klein
...plus the Legion text-piece by Paul Levitz. The "First Five Hundred Issues" history is by E.Nelson Bridwell, who ends the piece with "...and now we have #500. What next?"