The L5 Coaxial‐Carrier Transmission System is the first long‐haul, high‐capacity transmission system for which the design was strongly influenced by extensive application of computer‐aided design (cad) techniques. A tight time schedule required a parallel effort of (i) improving and using the somewhat limited capabilities of existing cad programs and (ii) developing new programs having increased capability and efficiency. Similar development of computer‐controlled measurement techniques provided necessary device and component characterization and subsystem evaluation. The result is a powerful group of tools that are independently important, but whose combined use helped make possible the timely completion of the L5 system design. These tools, now “proved in,” will profoundly influence the next system design philosophy.