Are the phenomena of physics "discovered" or rather "constructed"? If physical phenomena are to be understood as constructions, then physics is nothing more than useful fiction, lacking relevance for the characterisation of such a thing as -real- nature. This book puts forward two theses: First, that astroparticle physics is a very appropriate field for examining this issue; and second, that scientific research would unfold differently, depending on whether such research follows a realistic or a constructivist dynamics. In addition to the presentation of the key elements of astroparticle physics and the summary of evidence for the evaluation of the realistic and social constructivist positions at the present state of this discipline, this book provides some specific examples of research developments that will evolve in different ways in the near future, depending on whether astroparticle physicists pursue their research in the sense of realism or in the sense of social constructivism."