We study dynamic communicating automata (DCA), an extension of classical communicating finite-state machines that allows for dynamic creation of processes. The behavior of a DCA can be described as a set of message sequence charts (MSCs). While DCA serve as a model of an implementation, we propose branching high-level MSCs (bHMSCs) on the speci fication side. Our focus is on the implementability problem: given a bHMSC, can one construct an equivalent DCA? As this problem is undecidable, we introduce the notion of executability, a decidable necessary criterion for implementability. We show that executability of bHMSCs is EXPTIME-complete. We then identify a class of bHMSCs for which executability eff ectively implies implementability.