
The International Brigades were military units set up by the Communist International to assist the Popular Front government of the Second Spanish Republic during the Spanish Civil War. The organization existed for two years, from 1936 until 1938. It is estimated that during the entire war, between 40,000 and 59,000 members served in the International Brigades, including 15,000 who died in combat.

The 20th International Battalion was a battalion of the International Brigades during the Spanish Civil War.
The 35th Division was a division of the Spanish Republican Army in the Spanish Civil War.
The 45th Division was a division of the Spanish Republican Army in the Spanish Civil War.
The 86th Mixed Brigade, was a mixed brigade of the Spanish Republican Army in the Spanish Civil War. It was formed in March 1937 with battalions of the Carabineros corps. Until its demise it operated mostly in the Sierra Morena area, at the southern limits of the territory held by the Republic.

The American Medical Bureau (AMB), also known as the American Medical Bureau to Save Spanish Democracy, was a humanitarian aid institution associated to the Lincoln Battalion, providing a medical corps, nursing systems for casualties, as well as accommodation and treatment to those who were wounded on the Spanish Republican side of the battlefield in the Spanish Civil War (1936–1939).

The CL International Brigade or 150th International Brigade, also known as "Dabrowski Brigade", was a military unit of the International Brigades during the Spanish Civil War. Its members were mostly Polish, but there was also a Spanish battalion, as well as a Hungarian section.

The Connolly Column was the name given to a group of Irish republican socialist volunteers who fought for the Second Spanish Republic in the International Brigades during the Spanish Civil War. They were named after James Connolly, the executed leader of the Irish Citizen Army. They were a company-strength unit of the XV International Brigade, which also included the US, British and Latin American battalions in Spain. The name is now retroactively applied to all Irish volunteers who fought for the Spanish Republic.

The CXXIX International Brigade or 129th International Brigade was a military unit of the International Brigades during the Spanish Civil War. Its members were from a number of different countries thus it was also known as "Forty Nations Brigade".

The Dabrowski Battalion, also known as Dąbrowszczacy, was a battalion of the International Brigades in the Spanish Civil War. It was initially formed entirely of volunteers, "chiefly composed of Polish miners recently living and working in France and Belgium". Because of the relatively short travelling distances, these men were amongst the first to arrive in Spain. The battalion had a strong Polish flavour and even when, towards the end of the war, Poles were heavily outnumbered by Spanish troops, the officers and non-commissioned officers were still predominantly Polish. It also contained a significant nucleus of Red Army officers. It fought from 1936-1939. . The battalion was raised in Albacete in mid-October 1936.

The Dimitrov Battalion was part of the International Brigades during the Spanish Civil War. It was the 18th battalion formed, and was named after Georgi Dimitrov, a Bulgarian communist and General Secretary of the Comintern in that period.

The Gastone Sozzi Centuria was a Communist unit raised in Barcelona by Italian and Catalan volunteers who, at the beginning of the Spanish Civil War, went to the defence of Madrid, besieged by the Rebel Army. The name of the unit was in honour of Italian communist Gastone Sozzi, assassinated in 1928 by Fascist Blackshirts.

The International Brigade Memorial Trust is a British educational trust formed by the veterans of the International Brigade Association, the Friends of the IBA, representatives of the Marx Memorial Library, and historians specialising in the Spanish Civil War.

The Lincoln Battalion was the 17th battalion of the XV International Brigade, a mixed brigade of the International Brigades also known as Abraham Lincoln Brigade. It was organized by the Communist International with great care to conceal or minimize the communist character of the enterprise. Attractive names were deliberately chosen for the International Brigades, such as "Garibaldi" in Italy or "Abraham Lincoln" in the United States.

The Mackenzie–Papineau Battalion or Mac-Paps were a battalion of Canadians who fought as part of the XV International Brigade on the Republican side in the Spanish Civil War in the late 1930s. Except for France, no other country had a greater proportion of its population volunteer in Spain as did Canada. The XV International Brigade, made up also of volunteer battalions from the United States and Britain, was involved in the Battle of Jarama, in which nine Canadians are known to have been killed.

The International Brigades (IB) were volunteer military units of foreigners who fought on the side of the Second Spanish Republic during the Spanish Civil War. The number of combatant volunteers has been estimated at between 32,000–35,000, though with no more than about 20,000 active at any one time. A further 10,000 people probably participated in non-combatant roles and about 3,000–5,000 foreigners were members of CNT or POUM. They came from a claimed "53 nations" to fight against the Spanish Nationalist forces led by General Francisco Franco and assisted by German and Italian forces.

The Ukrainian interbrigade company Taras Shevchenko was a Ukrainian formation, which participated in the Spanish Civil War on the Republican side. It was composed of the Ukrainian citizens of Poland, who were members of the Communist Party of Western Ukraine.

Unlikely Warriors: The British in the Spanish Civil War and the Struggle Against Fascism is a history book by Richard Baxell about British people who served in the International Brigades during the Spanish Civil War. It was published by Aurum Press in 2012.

The XI International Brigade fought for the Spanish Second Republic in the Spanish Civil War.

The XII International Brigade was mustered on 7 November 1936 at Albacete, Spain. It was formerly named the Garibaldi Brigade, after the most famous and inspiring leader in the Italian Independence Wars, General Giuseppe Garibaldi.

The 13th International Brigade – often known as the XIII Dąbrowski Brigade – fought for the Spanish Second Republic during the Spanish Civil War, in the International Brigades. The brigade was dissolved and then reformed on four occasions.

The XIV International Brigade was one of several international brigades that fought for the Spanish Second Republic during the Spanish Civil War.

The Abraham Lincoln Brigade, officially the XV International Brigade, was a mixed brigade that fought for the Spanish Republic in the Spanish Civil War as a part of the International Brigades.

The Yankee Squadron was a group of mercenary American military aviators who flew for the Spanish Republican Air Force, during the Spanish Civil War.

The Yugoslav volunteers in the Spanish Civil War, known as Spanish fighters and Yugoslav brigadistas, was a contingent of volunteers from the Kingdom of Yugoslavia that fought for the Republicana during the Spanish Civil War (1936–1939). An estimated 1,664 "Yugoslav brigadistas" fought in the war, out of whom c. 800 were killed in action. According to Spanish statistics, 148 Yugoslav volunteers received the officer rank during the conflict.
