This paper presents market-based workflow management, a novel approach to workflow specification and execution which regards activities contained in a workflow as goods traded on an electronic market. Information about expected cost and execution time is considered for activity specifications, and is used at runtime to execute workflows such that actual cost and execution times are balanced and optimized. To that end, task assignment uses a bidding protocol, in which each eligible processing entity specifies at which price and in which time interval he/she can execute the activity. The winner of a specific bidding process is requested to execute the activity, and earns the amount specified in the corresponding bid. Market-based workflow management thus not only allows to optimize workflow executions with respect to execution time and overall cost; but the trading of activities also represents an incentive for processing entities to engage in a workflow.