A treaty series is an officially published collection of treaties and other international agreements.
League of Nations
The League of Nations Treaty Series (LNTS) was a result of article 18 of the Covenant of the League of Nations, which stated:
Every treaty or international engagement entered into hereafter by any Member of the League shall be forthwith registered with the Secretariat and shall as soon as possible be published by it. No such treaty or international engagement shall be binding until so registered.
The League of Nations Treaty Series began publication in 1920, and was terminated in 1946, following the dissolution of the League of Nations. It contains 205 volumes with a total of 4834 treaties. A 9-volume index has been created.
The United Nations Treaty Series (UNTS) is the result of article 102 of the Charter of the United Nations, which states as follows:
1. Every treaty and every international agreement entered into by any Member of the United Nations after the present Charter comes into force shall as soon as possible be registered with the Secretariat and published by it.
2. No party to any such treaty or international agreement which has not been registered in accordance with the provisions of paragraph 1 of this Article may invoke that treaty or agreement before any organ of the United Nations.
On December 14, 1946, the United Nations General Assembly passed resolution 97, which laid the rules for the registration of international agreements by the UN Secretariat.<"http://www.un.org/documents/ga/res/1/ares1.htm/">
The UNTS is by far the largest collection of treaties, running to some 2500 volumes containing 158,000 treaties from 1946. Its predecessor was the League of Nations Treaty Series (LNTS).
Treaties and international agreements were formally published in Statutes at Large (Stat.) until 1948. The Department of State also published a number of collections relating specifically to treaties and other agreements:
- Foreign Relations of the United States (FRUS), published under various names since 1861
- Treaty Series (TS or USTS), issued singly in pamphlets until 1945
- Executive Agreement Series (EAS), issued singly in pamphlets until 1945
- Treaties and Other International Acts Series (TIAS) are a series of pamphlets or "slip" treaties published by the Department of State to replace the Treaty Series and the Executive Agreement Series
- United States Treaties and Other International Agreements (UST) from accumulated TIAS pamphlets, published annually from 1950
- Foreign Relations of the United States (FRUS) continues to be published as a collection
For convenience, a number of private collections of treaties have also been published:
- Treaties and Other International Agreements of the United States of America 1776–1949 (Bevans), compiled by Charles I. Bevans
- International Legal Materials (ILM) published by The American Society of International Law (ASIL) 
- Treaties and other International Acts of the United States of America (Miller), edited by David Hunter Miller
United Kingdom treaties and international agreements are published as Command Papers, along with a large number of other government documents. For ease of reference, Command Papers relating to treaties are given a second "Treaty Series" number, which is sometimes referred to as the United Kingdom Treaty Series (UKTS). For example, the Treaty on Open Skies was published as Cm. 5537, but also had the Treaty Series number 26 for 2002, abbreviated as  UKTS 26.
Treaties signed by the Canadian government are published in a government publication titled Canada Treaty Series.
New Zealand treaties are published in New Zealand Treaty Series (NZTS), starting publication from 1944.
Republic of Ireland
The Irish Department of Foreign Affairs has published a list of all treaties of Ireland on its website, titled Irish Treaty Series. At present, all treaties from 2002 are available online.
The State of Israel has its own official collection of treaties, titled in Hebrew "Kitvey Amana" ("Treaty Writings" in Hebrew, also referred to as "Israel Treaty Series"). It includes every international agreement either signed or acceded by the Israeli government. 
The University of South Pacific in Port Vila, Vanuatu began a publication titled Pacific Islands Treaty Series, giving the texts of treaties concluded by small states in the Pacific Ocean.
- United Nations General Assembly resolution 24 (I) of 12 February 1946; League of Nations Assembly resolution of 18 April 1946 (League of Nations, Official Journal, Special Supplement No. 194, p. 57).
- Information on Canadian treaty procedures (from the website of the Canadian Department of Foreign Affairs)
- Irish Treaty Series online
- Partial list (in Hebrew) of treaties signed by the Israeli government (at the website of the Israeli Ministry of Justice)
- Search engine for Israeli bilateral treaties (at the website of the Israeli Ministry of Foreign Affairs)
- United States Treaties and International Agreements: 1776-1949 (edited by Charles I. Bevans)
- United States Treaties and Other International Acts Series online (from 1996 onward)
- Vol. 1 in League of Nations Treaty Series
- League of Nations Treaty Series online
- Text of vol. 1 in United Nations Treaty Series
- UK Treaty Series online
- Canadian Treaty Series online
- Australian Treaty Series online
- New Zealand Treaty Series online
- Pacific Islands Treaty Series online
- Council of Europe Treaty Series online
- Indian Treaty Series online