From the Central Bank :
Minimum Reserve System
The ECB requires that credit institutions hold compulsory deposits on account with the NCBs: these are called “minimum” or “required” reserves. The amount of reserves required to be held by each institution is determined by its reserve base, which is defined in terms of liability items on its balance sheet and includes deposits and debt securities issued. However, liabilities with respect to other institutions subject to the minimum reserve system are not included. Reserve requirements are based on a reserve ratio, currently 2 per cent.
In order to pursue the aim of stabilising interest rates, the Eurosystem’s minimum reserve system enables institutions to make use of averaging provisions. Compliance with reserve requirements is determined on the basis of an institution's average daily reserve holdings over a one-month maintenance period. Institutions' holdings of required reserves are remunerated at a rate which is based on the main refinancing operations rate, and thus close to market rates, although balances in excess of requirements are not remunerated. When an institution fails to meet its minimum reserve obligations sanctions may be imposed by the ECB.