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 since these methods and models are too complex and/or require input from the decision maker which makes him feel uncomfortable. The result is that most decision makers turn their back on scientific approaches and prefer to use common sense and `rules of thumb' when facing decision problems. To breach the current wide gap between decision makers and academic researchers a clear idea as to what a human decision maker is and is not capable of is needed. Based on these elements less sophisticated models need to be build. 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. Field cases are used to illustrate the methods and a lot of application fields are presented.