Architectural hierarchy: The building block for describing the system architecture is an agent that communicates with its environment via shared variables and also communication channels. The language supports the operations of composition of agents for concurrency, hiding of variables for information encapsulation, and instantiation of agents for reuse.

Behavioral hierarchy: The building block for describing a flow of control inside an atomic agent is a mode. A mode is basically a hierarchical state machine, that is, a mode can have submodes and transitions connecting them. Variables can be declared locally inside any mode with standard scoping rules for visibility. Modes can be connected to each other through well-defined entry and exit points. We allow the instantiation of modes so that the same mode definition can be reused in multiple contexts. Finally, to support exceptions, the language allows group transitions from default exit points that are applicable to all enclosing modes, and to support history retention, the language allows default entry transitions that restore the local state within a mode from the most recent exit.

Discrete and Continuous variable updates: Discrete updates are specified by guarded actions labeling transitions connecting the modes. Such updates correspond to mode-switching, and are allowed to modify variables through assignment statements.

Variables in CHARON can be declared as type analog, and they flow continuously during the continuous updates that model passage of time. The evolution of analog variables can be constrainted in three ways: differential constraints (e.g. by equations such as x' = f(x,u)), algebraic constraints (e.g. by equations such as y = g(x,u)), and invariants (e.g. x-y < c) which limit the allowed durations of flows. Such constraints can be declared at different levels of the mode hierarchy.