  Hierarchical Task Network (HTN) planning is a practical and efficient
approach to planning when the 'standard operating procedures' for a domain are
available. Like Belief-Desire-Intention (BDI) agent reasoning, HTN planning
performs hierarchical and context-based refinement of goals into subgoals and
basic actions. However, while HTN planners 'lookahead' over the consequences of
choosing one refinement over another, BDI agents interleave refinement with
acting. There has been renewed interest in making HTN planners behave more like
BDI agent systems, e.g. to have a unified representation for acting and
planning. However, past work on the subject has remained informal or
implementation-focused. This paper is a formal account of 'HTN acting', which
supports interleaved deliberation, acting, and failure recovery. We use the
syntax of the most general HTN planning formalism and build on its core
semantics, and we provide an algorithm which combines our new formalism with
the processing of exogenous events. We also study the properties of HTN acting
and its relation to HTN planning.
