Software system engineering has not yet developed an engineering science for its discipline. On the other hand, a lot of fundamental concepts, shared methods, techniques, patterns for structuring software systems, and languages for documenting design decisions has been accumulated over the years. To analyse and systematise the accumulated ideas is the main challenge for computer scientists today. The main objective of this paper is to analyse software system engineering both as a discipline and as an engineering science. A special attention is paid to conceptual modelling formalisms used in software system engineering.