Solar-terrestrial research has evolved rapidly in the 20 years since Sputnik 1 and Explorer 1 heralded the space age. Accelerating activity in space by many nations has led to considerable observational maturity in all facets of the field—solar, solar wind, magnetospheric, ionospheric, and upper atmospheric research. The directions of research have changed as exploration progressed. Two strong trends currently are evident: (1) more emphasis on the coupling of the various regions of space studied, and (2) more emphasis on physical processes that operate in these regions. Both of these trends represent a movement of solar-terrestrial research out of phenomenology into quantitative studies. As might be predicted, the trends are accompanied by increased activity in modeling and intensified demand for theoretical work concomitant with more comprehensive experimental observations.