A committee of inquiry is one of the most important means by which citizens can review the work of the Senate. The establishment of such a committee requires the support of one fifth of the parliamentarians - that is, 25 of the 123 members of parliament.
Its members have considerable power because the rules of the Parliamentary Investigation Committees (PUA) are based on the Code of Criminal Procedure: witnesses must appear before the committee and may only refuse to testify if they would thereby incriminate themselves.
Two parliamentary committees of inquiry have been set up for the period 2011 to 2015: The “Elbphilharmonie” PUA, that was appointed to investigate the construction delays and increased costs in building the Elbphilharmonie in HafenCity, and the PUA “Clarification of the neglect by the public authorities of the requirement to safeguard the best interests of the child in the case of Yagmur and development of recommendations for improving child protection in Hamburg”.
Since the last elections to the Bürgerschaft in 2020, the Parliamentary Committee of Inquiry "Cum-Ex Tax Revenue Scandal" has been set up. It deals with the question of "why the Hamburg Senate and the Hamburg tax authorities were prepared to let taxes amounting to millions lapse in respect of cum-ex transactions and to what extent this resulted in influence being exerted in favour of the tax-liable bank and to the detriment of the people of Hamburg".