Sissi is a Finnish term for light infantry which conducts reconnaissance, sabotage and guerrilla warfare operations behind enemy lines.

The word sissi, first attested in the modern meaning "patrolman, partisan, spy" in 1787, comes to Finnish from Slavic and refers either to a forest bandit or his yew bow.

[1] The Finnish Army trained Sissi units to conduct long range reconnaissance patrols, gather intelligence from concealed observation posts, raid enemy installations (especially supply depots), conduct road side ambushes and pursue and destroy enemy special forces units.

In wartime, an unspecified number of reservists were to be assigned to Sissi battalions and would deploy and operate as small groups up to company size.

Sissi battalions were part of Finnish Army "local troops", unlike the jäger and armored brigades meant for operational use.

Before the hostilities of Winter War, the Finnish Borderguard formed 25 Independent Battalions (Erillinen Pataljoona) from local reservists along the border area.

For example, the 4th Independent Battalion was directly under the command of Supreme Headquarters doing LRRP and raiding missions deep inside Soviet area.

In the Battle of Ilomantsi, soldiers of the 4th disrupted the supply lines of the Soviet artillery, preventing effective fire support.

Battles were short clashes of lightly armed infantry groups from squadron to battalion in size, with little chance of support or reinforcements.

However, the FDF paratroopers of the Utti Jaeger Regiment are trained in woodland area fighting, survival skills, unconventional methods and asymmetric tactics though the major role is geared more towards long range reconnaissance and special operations.