Modelling of complex systems with Message Sequence Charts requires several extensions in order to arrive at sufficiently transparent and manageable descriptions. Two extensions of major importance are introduced within this paper: on the one side, extended High Level MSCs denoted as HyperMSCs are allowed to contain MSC references with hypertext-like inscriptions or in expanded form as detailed MSCs; on the other side, MSC connectors are introduced in form of MSCs representing high level communication patterns between MSC components. The introduction of MSC connectors may be viewed as a generalisation of the gate concept and as a completion of the MSC language for communicating operator expressions. Moreover, MSC connectors can be employed quite generally as a communication description on a higher level of abstraction - a structural language construct which obviously is still missing in the MSC standard. These new concepts allow a system modelling based on stepwise refinement starting with HyperMSCs, decomposed instances and MSC connector communication