Subdwarf B stars (sdB) dominate the populations of faint blue stars and are found in both the old disk (field sdBs) and halo populations (globular cluster members) of our own Galaxy. The origin of sdB stars remains largely a puzzle, but evidence is accumulating that close binary evolution is fundamental to the problem. With interferometric methods, we expect to detect and learn about close cool companions to derive further insight in the evolution of sdB stars.