An apparatus (10; 10') for processing bank notes, wherein the apparatus (10; 10') comprises: an input pocket (22; 22') for inputting bank notes; at least one output pocket (36a-e) for outputting bank notes; a first mechanical interface (46a, b) and a second mechanical interface (56a, b) for detachably connecting cassettes (70a, 70b) for bank notes; a transport system (24, 34, 44, 54) which is configured for transporting bank notes from the input pocket (22; 22') to the at least one output pocket (36a-e); and transporting bank notes from a cassette (70a, 70b) detachably connected with the apparatus (10; 10') via the first or second mechanical interface (46a, b, 56a, b) to the at least one output pocket (36a-e), characterized in that the apparatus (10; 10') is constructed in modular fashion; the apparatus (10; 10') comprises a base module, an output module, a cassette emptying module and a cassette filling module, the base module and the cassette emptying module being connected via a module connecting unit, one cassette is detachably connected with the cassette emptying module via a first mechanical interface (46a, b), the first mechanical interface (46a, b) being part of the cassette emptying module, and one cassette is detachably connected with the cassette filling module via the second mechanical interface (56a, b), the second mechanical interface (56a, b) being part of the cassette filling module; the transport system (24, 34, 44, 54) is further configured for transporting bank notes from the cassette detachably connected with the apparatus (10; 10') via the first mechanical interface (46a, b) to the at least one output pocket (36a-e), the output pocket (36a-e) being part of the output module, for transporting bank notes from the input pocket (22; 22') into a cassette detachably connected with the apparatus (10; 10') via the second mechanical interface (56a, b), the input pocket (22; 22') being part of the base module, and for transporting bank notes from the cassette detachably connected with the apparatus (10; 10') via the first mechanical interface (46a, b) into the cassette detachably connected with the apparatus (10; 10') via the second mechanical interface (56a, b), and the first and second mechanical interfaces (46a, 46b, 56a, 56b) have a latch. an input pocket (22; 22') for inputting bank notes; at least one output pocket (36a-e) for outputting bank notes; a first mechanical interface (46a, b) and a second mechanical interface (56a, b) for detachably connecting cassettes (70a, 70b) for bank notes; a transport system (24, 34, 44, 54) which is configured for transporting bank notes from the input pocket (22; 22') to the at least one output pocket (36a-e); and transporting bank notes from a cassette (70a, 70b) detachably connected with the apparatus (10; 10') via the first or second mechanical interface (46a, b, 56a, b) to the at least one output pocket (36a-e), characterized in that the apparatus (10; 10') is constructed in modular fashion; the apparatus (10; 10') comprises a base module, an output module, a cassette emptying module and a cassette filling module, the base module and the cassette emptying module being connected via a module connecting unit, one cassette is detachably connected with the cassette emptying module via a first mechanical interface (46a, b), the first mechanical interface (46a, b) being part of the cassette emptying module, and one cassette is detachably connected with the cassette filling module via the second mechanical interface (56a, b), the second mechanical interface (56a, b) being part of the cassette filling module; the transport system (24, 34, 44, 54) is further configured for transporting bank notes from the cassette detachably connected with the apparatus (10; 10') via the first mechanical interface (46a, b) to the at least one output pocket (36a-e), the output pocket (36a-e) being part of the output module, for transporting bank notes from the input pocket (22; 22') into a cassette detachably connected with the apparatus (10; 10') via the second mechanical interface (56a, b), the input pocket (22; 22') being part of the base module, and for transporting bank notes from the cassette detachably connected with the apparatus (10; 10') via the first mechanical interface (46a, b) into the cassette detachably connected with the apparatus (10; 10') via the second mechanical interface (56a, b), and the first and second mechanical interfaces (46a, 46b, 56a, 56b) have a latch.