Conference of Solidarity Support Organizations (CSSO) was an international organization of national groups that supported the trade union Solidarność and human rights in Poland during the final years of the Polish People's Republic. Member organizations agreed to the CSSO Letter of Agreement of January 8/9, 1983, and there were eventually more than twenty international chapters.
 The CSSO emerged from a series of informal interorganizational meetings throughout 1982 among Solidarity support organizations.  To provide a formal basis for cooperation, and to increase the unity of these support organizations, organizers drafted an agreement. To join the CSSO, organizations had to ratify the CSSO Letter of Agreement.  In it, groups agreed to support and recognize Solidarity in Poland, along with the Coordinating Office Abroad of NSZZ Solidarnosc (based in Brussels, Belgium).  Member organizations still had the freedom to support their own initiatives in Poland and were not required to support the Brussels office, nor proscribe any other links or channels with Solidarity. The CSSO ultimately included 46 organizations in 13 countries.[citation needed]
 The CSSO was designed as a forum in which member organizations could exchange ideas and share experiences pertaining to protest activities in Poland. The organization held meetings in which member representatives would consider joint CSSO resolutions and actions.  The CSSO did not control the activities or funds of the member organizations. .
 The greatest impact of the CSSO was in fostering trust among the leaders of the member organizations.  This did indeed foster collaboration and thus improved the efficacy of numerous initiatives and programs.
 Most of the accomplishments came from the member organizations themselves. These accomplishments range from: a massive smuggling of materials to  Solidarity and other groups in Poland; to the above-ground Family-to-Family assistance to the families of the imprisoned and persecuted Solidarity activists; to the Independent Polish Agency news agency providing information and photographs to the news media across the globe; to the professional periodic publications such as Kontakt (Paris), Pogląd and Meinung (West Berlin), Voice of Solidarity (London), ARKA de Informaciones (Mexico City), and Porando-geppo (Tokyo); to political action, lobbying and raising awareness activities in every country; to assistance to the Brussels Coordinating Office and activists in their work on behalf of Solidarity; to sending financial resources to Solidarity and opposition groups in Poland.
 The membership of the CSSO organizations consisted primarily of the citizens of the countries where these organizations were located and usually included members who were not of Polish heritage. David Phillips, the first CSSO Secretary, was not of Polish ancestry.  In fact in some organizations, the majority were not of Polish heritage, though sometimes did include non-Poles who were nevertheless fluent in Polish – Yoshiho Umeda (CSSO Asian Coordinator), and Jan Axel Stolz, who served as CSSO Secretary.
 Papers and holdings of the CSSO have been preserved in countries where there were active chapters, or an active Polish diaspora population. This also includes Andre Blaszczynski's papers.