This book deals with the field of mathematics didactics research. Real research implies that we don't really know what we're going to find, or even if we're going to find it at all: such adjustments in the course of research are not to be regarded as failures, but as an integral part of the process.Research planning is what we call a "work plan", as opposed to a "writing plan". This is "planning", not "programming", and should always be open to revision in the light of unforeseen circumstances (difficulties, but also opportunities), and gradually refined as the research progresses.