Additionally, the University of Minnesota maintains a guide to:
Generally, treaties designate a depository- an organization or government responsible for keeping the text of the treaty and tracking which countries have signed, ratified, and made reservations to the treaty.
Virtually all modern treaties designate the United Nations as a depository. Additionally, even if a treaty does not formally designate the U.N. as a depository, Article 102 of the Charter of the United Nations requires all member nations to register all treaties with the United Nations. Article 18 of the Covenenant of the League of Nations (the U.N.'s precursor) also required all member nations to register all treaties with the League of Nations.
Accordingly, the overwhelming majority of modern treaties are published in:
The U.N. also publishes a convenient summary of the current status of each multilateral treaty in:
A few early international law scholars undertook projects to compile print collections of historical multilateral treaties. Each collection is generally referred to by the name of its compiler (e.g. Parry, Martens, and Dumont.)
The most comprehensive collection is Parry, available in print at the library:
Parry was explicitly intended as a replacement and update for two earlier treaty series (Martens and Dumont) which were not as complete and contained treaties primarily in their original language or French. Complete scans of both Martens and Dumont are available on HeinOnline: