Who are they?
The FDP, formed under Theodor Heuss in 1948, began as an amalgam of the remains of pre-war liberal parties. The party is seen as traditionally liberal, favouring open, unregulated markets, minimal government intervention and the protection of individual rights and freedoms making it liberal both economically and socially.
They held the balance of power in the last elections in 2009 by forming a coalition with the Christian Democrats (CDU) and will be hoping to do the same this year.
The FDP describes itself as the “party of the centre”, appealing to voters not identifying with the socially conservative CDU or the economically centre-left SPD.
Who's their leader?
Former party leader and incumbent foreign minister Guido Westerwelle was replaced in 2011 by Philipp Rösler, also serving as federal minister for economy and technology.
Born in Vietnam, Rösler is the first Asian-born cabinet minister in Germany. He was adopted by a German couple and came to West Germany aged nine months.
Rösler trained to become a medic in the German army after he left school and went on to study medicine in Hannover. The 40-year-old joined the youth branch of the FPD in 1992 and became secretary of the FDP in Lower Saxony in 2000.
In 2008 he was elected to stand as his party's main candidate in the Lower Saxony state election and was appointed deputy prime minister of the state of Lower Saxony.
A year later his rapid rise continued and he became the health minister in Angela Merkel's cabinet. In 2011 he was named economy minister, German vice-chancellor and chairman of the FDP.
What's their strategy?
The pro-business FDP have seen their free market economic policies given more leash than usual during their partnership with the CDU, and naturally look to continue in the coalition. But forming the coalition has been a disaster for their poll ratings.
Westerwelle, who led the party to its record result in the 2009 election, made the party unpopular with voters with its aggressive free market policies, demanding tax cuts and attacking welfare.
The FDP is now struggling to poll at much more than five percent of the vote – the amount needed to get into parliament.
Why do they matter?
If the FDP's dangerously low poll ratings are represented in the election results, the CDU's lead over a potential SPD-Green coalition would be in jeopardy, forcing Merkel's party to find new coalition partners or discuss a grand coalition - an unappetizing option for parties offering different political agendas.
If it does clear the five percent threshold then the FDP will find itself in power once again despite having so little popular support.
The Christian Democratic Party (CDU)