A review of the status of weather-modification research, with emphasis on progress since 1966. The authors stress that 1) the possibility of inadvertent weather or climate modification is rapidly becoming a probability, as human effects on the atmosphere and the surface of the planet grow at an increasing rate; 2) progress in weather modification research continues to be hampered by the prevalent lack of cohesive effort by both theoreticians and experimenters; 3) computers of advanced design and increased capacity will handle atmospheric models of considerably greater sophistication than in the past; and 4) this is a not-to-be-neglected opportunity for interactive research—constant two-way feedback from theory to experiment to theory, with dynamic atmospheric models facilitating each advance. General and specific recommendations are appended concerning areas most urgently requiring research and instrumentation most drastically needing development.