Providing flexible transaction semantics and incorporating activities, data and agents are the key issues in workflow system development. Unfortunately, most of the commercial workflow systems lack the advanced features of transaction models, and an individual transaction model with specific emphasis lacks sufficient coverage for business process management.This report presents our solutions to the above problems in developing Open Process Management System (OPMS) at HP Labs. OPMS is based on nested activity modeling with the following extensions and constraints: in-process open nesting} for extending closed/open nesting to accommodate applications that require improved process-wide concurrency without sacrificing top-level atomicity; confined open as a constraint on open and in-process open activities for avoiding the semantic inconsistencies in activity triggering and compensation; and two-phase remedy as a generalized hierarchical approach for handling failures.