This book is a contribution to the definition of a model based system engineering (MBSE) approach which would meet the objectives laid out by the INCOSE . After a reminder about complexity that jeopardizes a lot of system developments and the MBSE objectives to face this complexity, the book addresses fundamental ontological and epistemological aspects of systems under consideration (technological systems, knowledge and models). Afterwards, it addresses methodological issues and proposes a methodic approach of MBSE that provides, at the opposite of current practices , systematic and integrated model based engineering processes. An annex describes relevant features of the VHDL-AMS language supporting the methodological issues described previously.