This paper presents market-based workflow management, a novel approach to work-flow specification and execution which regards tasks performed in a workflow as goods traded in 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 follows a bidding protocol, in which each eligible processing entity specifies at which price and in which time interval it 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 allows to optimize workflow executions with respect to execution time and overall cost; additionally the trading of activities provides an incentive for processing entities to engage in a workflow.