Illegal Jewish Immigration to Mandatory Palestine during the British mandate 1934- 1948.
During this period 20,000 Jews immigrated to Palestine without British mandate certificates. The majority arrived on illegal ships from Europe, while others came overland from Arab countries.
Two important events initiated the start of Illegal Immigration to Mandatory Palestine: the rise of anti-Semitism in Europe caused many Jews to leave their countries; British policies limited the number of legal immigrants, which caused institutions, organizations and private citizens to arrange transport for the immigrants to Palestine. As a result, Yishuv leaders decided to establish the Hamossad For Aliya Bet in 1939, headed by Shaul Me'erov Avigur.

Immigration via sea
Hagana members had a number of different roles in Hamossad For Aliya Bet:
Jewish volunteers in the British army, serving in Europe, secretly assisted holocaust survivors, arranged their transport as part of Habricha (The Escape) Organization- escape from Europe and transported them to ports of exit. The Palyam, the Plamach's naval company, safeguarded the vessels en route to Palestine.
The Gideonim- wireless operators, who maintained communications between the ships and Mossad outposts in Europe and in Palestine;
The Hagana assisted in the embarkation of the immigrants from the ships, hiding them from the British.
The Jewish Diaspora, and US Jews in particular, donated money for the purpose of purchasing ships and staff.
Exodus - 1948 - was the most famous illegal immigration ship. It sailed from France on 11.7.47, with 4,515 immigrants on board. The British overtook her near the shores of Palestine, and sent the immigrants back to Europe. Hagana members encouraged the immigrants to refuse to disembark in France. The British redirected the ship to Hamburg Germany and forced the immigrants off the ship, which as a result, increased international support for the opening the gates of Palestine to immigration and Jewish independence.
During 1945-1948, 65 illegal immigration ships, carrying on board 70,000 immigrants, arrived in Palestine.

Immigration via land
Hamossad For Aliya Bet was responsible for the Jewish immigration from Arab countries, especially from Iraq and Syria. The Chulia -The Detachment, a Palmach unit that assisted in smuggling immigrants, helping them cross the northern border and distributing them among the settlements in the Galilee.
5,000 illegal immigrants arrived in Palestine during 1942-1947 via land and air.
100 illegal immigrants from Iraq and 50 from Italy were flown to Palestine in 1947.
On the eve of the War of Independence, immigrants comprised 1/6 of the Yishuv population; some arrived during the War of Independence, after their release from British detention camps in Cyprus.