The Bundesrat is the forum through which the 16 Länder participate in the legislative process and administration of the Federation (see Art. 50 of the Basic Law). As a matter of principle, the Bundesrat participates in the passage of every law adopted by the Bundestag. The extent of its participation, however, depends on whether the bill in question is one to which the Bundesrat may lodge an objection or one requiring the Bundesrat's consent.

In keeping with the Basic Law, it is more usual for bills not to require the consent of the Bundesrat in order to become law. In such cases, the Bundesrat may lodge an objection to the bill. Thus the Bundesrat has the right to lodge an objection to most bills adopted by the Bundestag. If the Bundesrat wishes to object to a bill it must first ask the Mediation Committee to convene (for the mediation procedure, see below). The Bundestag can overturn an objection from the Bundesrat by an absolute majority, i.e. a majority of its statutory members; in some special cases, what is known as a "double qualified majority" is required (see Art. 77 (4) of the Basic Law). Thus, by holding another vote, the Bundestag can overcome the opposition of the Bundesrat and open the way for the law to be promulgated.

Where specifically provided for in the Basic Law, a bill requires the consent of the Bundesrat to become law. The Bundesrat may therefore exercise an absolute veto in such cases: if it refuses to give its consent, then the bill has failed. The Bundestag cannot override this veto, no matter how large a majority of its Members supports the bill, and even if support for the bill is unanimous. A bill is considered to require the consent of the Bundesrat if it substantially affects the interests of the Länder. A bill may fall into this category if it affects the finances of the Länder or if it has a particular effect on the Länder's execution of legislation, the organisation of the Land administrative authorities or the implementation of any other measures by the Länder. The reform and modernisation of the federal system which took place in 2006 modified the provisions governing which bills require the Bundesrat's consent. Since this point, the number of bills requiring the consent of the Bundesrat has diminished considerably; yet these bills still constitute almost fifty per cent of bills promulgated.

The question of whether the Bundestag is concerned with a bill to which the Bundesrat may lodge an objection or for which its consent is required is thus a key aspect of the legislative process and may determine whether the Bundestag in fact succeeds in having a bill passed. It is, however, not always easy to determine into which category the bill falls. Disagreements on this issue are not uncommon between the Federal Government and the Bundestag on the one hand and the Bundesrat on the other. The Federal Constitutional Court has had to decide more than once whether a bill requires, or should have required, the consent of the Bundesrat. If the Bundesrat fails to give its consent to a bill of this kind, then the bill becomes null and void.

In the above example, there was no doubt that this was a bill requiring the consent of the Bundesrat, since it is related to taxes from which some of the revenue goes to the Länder (Article 105, paragraph (3) of the Basic Law). However, the Bundesrat had substantive objections to the bill. Once the Bundestag had adopted the bill and, in keeping with Article 77, paragraph (1) of the Basic Law, had submitted it "to the Bundesrat without delay", the bill was discussed by the appropriate committees at the Bundesrat. During its 852nd plenary sitting of 5 December 2008, the Bundesrat concluded that it could not consent to the bill in the form proposed. The Bundesrat therefore decided to demand that the Mediation Committee be convened.

In the case of laws which require the consent of the Bundesrat, it is also possible for the Bundestag and the Federal Government to demand that the Mediation Committee be convened. The Bundesrat is not obliged to refer a bill to the Mediation Committee if it wants to defeat a bill by refusing to give its consent. Therefore, this provides the Bundestag and the Federal Government with an instrument to prevent the defeat of a bill, through a process of mediation.