Continued Journey Into Mystery Volume 1
The numbering of this series was a continuation from Thor vol. 3. That series changed it's name back into "Journey into Mystery" starting with issue 622, tying into Fear Itself. The series featured Loki as the main character from that point on and did so untill issue 645, after which his story was continued in Young Avengers Vol.2. Thor got a new ongoing series for himself when this series was started, namely The Mighty Thor (vol.2). From issue 646 the main character switched to Sif. The series lasted until issue 655, making it one of the longest collective series.
The name of this series comes from Journey into Mystery vol. 1 which was a comic that began publication in 1952 by Atlas, and although it wasn't until the eighty-third issue that Thor even appeared in comics, let alone the title, it is most often linked to his character. This is why after releasing the one hundred and twenty-fifth issue of the series in 1966 that the series changed its name to become the first volume entitled Thor. For more than three hundred issues the title remained as Thor, but in 1996 after the five hundred and second issue of the series, they decided to change the name back to Journey into Mystery which is what it was called up until its end in 1998 with the five hundred and twenty-first issue. Although these later Journey into Mystery issues (503-521) spin out of Thor vol.1, they are not counted in the collective Thor series.
Back in 1998, Thor's adventures restarted in the first issue of Thor vol. 2 and there they were contained until issue eighty-five when the series concluded because of Avengers: Disassembled in 2004. After that event Thor disappeared for a couple years until he returned in 2007 for his newest volume, Thor vol. 3 which only lasted for twelve issues with its restarted numbering. After that Marvel realized it had now published 599 comics in the collectively-grouped "Thor" series (even though eighty-two issues had nothing to do with him) and therefore reverted the numbering back to its original formation and thus, the 600th issue and the succeeding issues were born.
Non-U.S. Collected Editions