In the Kingdom of Poland, the term Sejm referred to an entire two-chamber parliament, comprising the Chamber of Deputies (Izba Poselska), the Senate and the King.

During the existence of the Polish People's Republic, the Sejm, then a unicameral parliament, was the supreme organ of state power in the country.

Sejm (an ancient Proto-Lechitic word meaning "gathering" or "meeting") traces its roots to the King's Councils – wiece – which gained authority during the time of Poland's fragmentation (1146-1295).

At the time, Poland's nobility, which accounted for around 10% of the state's population (then the highest amount in Europe), was becoming particularly influential, and with the eventual development of the Golden Liberty, the Sejm's powers increased dramatically.

The Sejm eventually became even more active in supporting the goals of the privileged classes when the King ordered that the landed nobility and their estates (peasants) be drafted into military service.

The Union of Lublin in 1569, united the Kingdom of Poland and the Grand Duchy of Lithuania as one single state, the Polish–Lithuanian Commonwealth, and thus the Sejm was supplemented with new envoys from among the Lithuanian nobility.

Its chambers reserved the final decisions in legislation, taxation, budget, and treasury matters (including military funding), foreign policy, and the confirment of nobility.

The 1573 Warsaw Confederation saw the nobles of the Sejm officially sanction and guarantee religious tolerance in Commonwealth territory, ensuring a refuge for those fleeing the ongoing Reformation and Counter-Reformation wars in Europe.

Additionally, if the envoys were unable to reach a unanimous decision within six weeks (the time limit of a single session), deliberations were declared void and all previous acts passed by that Sejm were annulled.

The constitution's acceptance, and the possible long-term consequences it may have had, is arguably the reason that the powers of Habsburg Austria, Russia and Prussia then decided to partition the Polish–Lithuanian Commonwealth, thus putting an end to over 300 years of Polish parliamentary continuity.

After the fall of the Duchy of Warsaw, which existed as a Napoleonic client state between 1807 and 1815, and its short-lived Sejm of the Duchy of Warsaw, the Sejm of Congress Poland was established in Congress Poland of the Russian Empire; it was composed of the king (the Russian emperor), the upper house (Senate), and the lower house (Chamber of Deputies).

The 64-member Senate on the other hand, was composed of voivodes and kasztelans (both types of provincial governors), Russian envoys, diplomats or princes, and nine bishops.

After numerous failures in securing legislative sovereignty in the early 19th century, many Poles simply gave up trying to attain a degree of independence from their foreign master-states.

On 2 September 1939, the Sejm held its final pre-war session, during which it declared Poland's readiness to defend itself against invading German forces.

On 2 November 1939, the President dissolved the Sejm and the Senate, which were then, according to plan, to resume their activity within two months after the end of the Second World War; this, however, never happened.

Meanwhile, in Nazi-occupied Poland, the Council of National Unity was set up; this body functioned from 1944 to 1945 as the parliament of the Polish Underground State.

With the cessation of hostilities in 1945, and subsequent rise to power of the Communist-backed Provisional Government of National Unity, the Second Polish Republic legally ceased to exist.

For instance, it was empowered with control over "the functioning of other organs of State authority and administration," and ministers were required to answer questions posed by deputies within seven days.

[1] Even though the Sejm was largely subservient to the Communist party, one deputy, Romuald Bukowski (an independent) voted against the imposition of martial law in 1982.