Abstract: Problems of combinatorial design of complex machining/assembly lines are considered. Operations are partitioned into groups which are performed either by a team of workers for manualy lines or by a piece of equipment for automated lines (for example by a multi-spindge head). Constraints related to the design of worker teams or spindle heads, working position and line configurations, as well as precedence constraints related to operations, are given. Such problems consist in minimizing the estimated cost of the corresponding machining/assembly line, while reaching a given cycle time and satisfying all constraints. A decision support system is developed. Several optimisation methods were implemented: MIP formulation, a constrained shortest path approach, Branch and Bound techniques, metaheurstics and heuristics. The developed decision-aid software tool is presented. Industrial examples are reported for different types of lines. Keywords: Machining lines, Assembly lines, Line design, Line balancing, combinatorial optimization, MIP, graph theory, metaheuristics, heuristics.