Presented in this paper is a systematic approach to "modeling and simulation of flow lines in mass production systems, using a tire trimming line as an example. The "modeling phase" consists of 1) generation of alternative line configurations, 2) construction of a reference model for each alternative, and 3) formal description of the target system. ACD(Activity Cycle Diagrams) are employed as a tool for formal description. In the "simulation phase. block diagram models (provided by the simulation language SIMAN) and the next event methodology(implemented in FORTRAN 77) are combined in order to fully describe the flow line behavior.