This note introduces and describes scenario-building, the strategic-thinking approach developed by Pierre Wack and associates at Royal/Dutch Shell in the 1970s and helped catapult the company from seventh to second in its industry. The note defines the concept, outlines who should use it and how it links to strategic planning, and illustrates it with models and verbal examples. The note is appropriate for use in courses on business economics, strategic thinking, business policy, and leadership.