Leśni (short for "leśni ludzie", Polish for "forest people") is an informal name applied to some anti-German partisan groups that operated in occupied Poland during World War II.
The "forest people" groups comprised mostly people who for various reasons could not operate from the communities they lived in and had to retreat into the forests.
Unlike most organized resistance groups, such as the Home Army, the "forest people" formed a sort of standing army, as opposed to regular partisan units that gathered shortly before an operation and afterward returned to their homes.
The first such groups formed in 1939, shortly after the invasion of Poland. Membership comprised mostly marauders from the Polish Army and other people who evaded arrest by the new Nazi or Soviet authorities. In March 1940 the partisan unit of the first guerrilla commander in the Second World War in Europe - Henryk Dobrzański "Hubal" - completely destroyed a battalion of German infantry in a skirmish near the village of Huciska. A few days later in an ambush near the village of Szałasy it inflicted heavy casualties upon another German unit. To counter this threat the German authorities formed a special 1,000-strong anti-partisan unit of combined SS and Wehrmacht forces, including a Panzer group. Although Major Dobrzański's unit never exceeded 300 men, the Germans fielded at least 8,000 men in the area to secure it.
Although most of the groups accepted the overall command of ZWZ and then of the Home Army, different realities made them remain largely independent.
In early 1943 the strength of the leśni groups can be estimated at about 40 groups numbering in total 1,200 to 4,000 fighters, but the numbers grew significantly during Operation Tempest in 1944.