Germans get two votes, one for a local candidate, and one for a party.
The Bundestag's 622 seats are divided between the two, with 299 going to local candidates (under a first-past-the-post system), and the remainder allotted to politicians from party lists, on a proportional representation basis.
To enter parliament, each party must secure a minimum of 5 percent of the vote. Votes given to parties that do not meet this threshold are redistributed amongst those who do.
German politics is all about forging coalitions and alliances; no single party has governed the nation alone since reunification.
Who will win?
Until recently, the election looked like a slam-dunk for the popular Merkel and her allies. However, the Free Democrats' dip in the polls puts the current coalition at risk.
Should the FDP lose its place in the Bundestag -- a possibility if the fledgling AfD splinters the conservative vote -- Merkel would have to cement fresh alliances in order to remain chancellor.
The CDU has ruled out linking up with the AfD -- despite their mutual conservatism, the AfD's resolutely anti-euro stance contradicts the Christian Democrats' policies.
Instead Merkel could return to the "grand coalition" days of 2005-2009, working with the Social Democrats -- and Steinbrueck -- once more.
Alternatively, should they perform well at the polls, the Social Democrats could seek to oust Merkel by forming a left-wing coalition with the Greens and The Left.
Why should those outside Germany care?
Germany is Europe's largest economy and its government has the leading voice in governing the Eurozone.
If German voters decide to replace conservative Merkel with left-leaning Steinbrueck, the whole currency union could be affected.
Steinbrueck is skeptical of Merkel's austerity-focused policies and has said he would do more to encourage growth and investment.
Although they disagree on economic policies, Steinbrueck has made it very clear that should he win the election, he would follow in Merkel's footsteps by demanding all eurozone countries stick to "sound fiscal discipline."