Decision theory renders the human decision maker the centre of attention: observing, describing how he makes decisions and leading him by the hand in an attempt to prevent him from taking 'illogical' decisions. Literature and scientific publications on decision theory should be accessible and understandable to the average decision maker but this is almost never the case. The result is that most decision makers turn their back on scientific approaches. In this book, a KISS-principle is proposed and several methods in the field of decision making by one decision maker or by group, resulting from close contacts with real world decision makers, are presented. KISS is a bacronym for the empirical principle 'Keep it simple, stupid'. KISS states that design simplicity should be a key goal and that unnecessary complexity should be avoided.