Operating a business is inevitably related to the obligation to operate a full accounting system. An enterprise may keep the accounts on its own or by hiring an accounting firm. The most often method to satisfy this obligation is to keep the accounts in the form of a revenue and expense ledger (PKPiR), i.e. so-called simplified accounting. The second method requiring more efforts and costlier is the full accounting system. Therefore, it is worth knowing what entities are obliged to operate such full accounting system and how one could prepare to do it.