A number of on-line repositories of scientific workflows are emerging. These repositories enable sharing and reuse of workflows, and aid in the design of new workflows. The growing size of these repositories, the complex hierarchical structure of the workflows, and the need to incorporate access control mechanisms make information discovery in these repositories an interesting challenge. This paper formalizes keyword search in repositories of hierarchical workflows. We start by defining what it means for a single hierarchical workflow to match a keyword query while accounting for access control, and discuss options for displaying the resulting matches within the workflow. We extend this to search over workflow repositories, by proposing various ranking semantics that build on techniques from XML search and from information retrieval, and adapting them to our setting.