The ParseTalk model of concurrent, object-oriented natural language parsing is introduced. It builds upon the complete lexical distribution of grammatical knowledge and incorporates inheritance mechanisms in order to express generalizations over sets of lexical items. The grammar model integrates declarative well-formedness criteria constraining linguistic relations between heads and modifiers, and procedural specifications of the communication protocol for establishing these relations. The parser's computation model relies upon the actor paradigm, with concurrency entering through asynchronous message passing. We consider various extensions of the basic actor model as required for distributed natural language understanding and elaborate on the semantics of the actor computation model in terms of event type networks (a graph representation for actor grammar specifications) and event networks (graphs which represent the actor parser's behavior). Besides theoretical claims, we present an interactive grammar/parser workbench, a graphical development environment with various types of browsers, tracers, inspectors and debuggers that has been adapted to the requirements of large-scale grammar engineering in a distributed, object-oriented specification and programming framework.