Activation policies for workflows are described. Some implementations support the configuring of a workflow such that an activity in the workflow can start processing items on which a prior activity's processing is completed, but before the prior activity has completed processing all the items. In addition, this activity can be configured to group the items received so far into a subset and process it according to the action of the activity and then send it to a next activity; and the activity can be configured to repeat this process multiple times using the items that have been received since the previous subset. In some implementations, an input set of items for a workflow is generated from a set of one or more sources, and the workflow is generated by splitting the input set into subsets via a recurring catch and release mechanism that is responsive to activation policies of the activities of the workflow. In addition, workflow visibility tracking information regarding the workflow is determined and caused to be displayed.