This textbook for teaching reasoning processes and decision making under conditions of uncertainty is written at an introductory level. Based on a translation of a Hebrew book, the text is intended for the general training of military and civilian personnel required to process, sort, and/or evaluate incomplete, unreliable information. Potential users are managers, commanders, and forecasting specialists. The first section, chapters 1 through 4, provides a general framework for thinking about uncertainty. Specific chapter topics include certainty and uncertainty, defining the uncertain situation, listing and grouping possible answers, and defining degrees of belief. The second section, chapters 5 through 7, discusses two tools, estimation and sampling, frequently used in dealing with problems caused by uncertainty. The last section, chapters 8 through 11, deals specifically with probability assessment. The relationship between the chances for a single observation of a characteristic, the percentage of that characteristic in a group, and estimation of chances in exceptional problems are covered. Two examples are presented that are worked out using the starting-point-and-adjustment method. Exercises are provided within each chapter. A list of technical references and an outline of related topics for further study conclude the text. (YLB)