In the midst of a changing economy, most executives continue to use a strategy toolkit designed for yesterday's more stable marketplace. As a result, strategies emerge that neither manage the risks nor take advantage of the opportunities that arise in highly uncertain times. Now, McKinsey shows strategists how to tailor every aspect of the decision-making process-from formulation to implementation-to the level of uncertainty faced, describes the strategic-planning processes readers can use to monitor, update, and revise strategies as necessary in volatile markets, and includes a toolkit for identifying, developing, and testing new strategy options-complete with guidelines for applying the right tool to the right situation at the right time. A comprehensive approach to strategy development under all possible levels of uncertainty and across all kinds of industries, this is the essential guide for making tough strategic choices in a changing world. Hugh Courtney is an Associate Principal with the Global Strategy Practice at McKinsey Company in Washington D.C.