A method and a system are provided for machining, especially for joining work pieces (2) in the shell of a body structure. The work pieces are transported in a continuous manner along a transfer line (3) by a conveyor (5) and are machined by several robots (7, 8) which are preferably arranged in a stationary manner on the transfer line. The robots (7, 8) are synchronized with the conveying movement of the workpieces (2). The movement and the position of the workpieces (2) are detected by a sensor system (13) which informs a control system (12) which controls the conveyors (5) and the robots (7,8). The machining system (1) can comprise a monitoring system (11) provided with an optical image detection system, which enables synchronization to be monitored and possibly retroactively adjusted.