The paper looks at those aspects of NBP accounting which will have to be changed when the Bank joins the European System of Central Banks. The changes will include new principles, procedures, reporting and computer systems. The author discusses the main accounting principles described by ESCB's guidelines and the prevailing accounting techniques, as well as methods to measure financial performance. Those principles and the fact that ESCB requires frequent reporting for the sake of both its operational and analytical activities makes it necessary to put in place an Integrated Accounting System. The system must possess certain features described by the author. Moreover, its implementation must be coordinated with organisational changes that should be provided for in advance. These include the introduction of bank account numbers complying with the international standard of new procedures for bank account servicing, and restructuring NBP's regional branch network.