A prototype knowledge-based system that helps select test methodologies for a particular type of logic structure is described. The system, called TDES, (testable design expert system), is a subsystem of Adam, an advanced design automation system. The system is being used to test programmable logic arrays, but its architecture is applicable to other types of structures such as RAMs, ROMs, and other combinational logic. It uses a divide-and-conquer (partitioning) strategy and works interactively with a user as an intelligent consultant and assistant.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>