We present a framework for monitoring information flow in security-critical reactive systems, such as communication protocols, cell phone apps, document servers and web browsers. The secrecy requirements in such systems typically vary over time in response to the interaction with the environment. Standard notions of secrecy, like noninterference, must therefore be extended by specifying precisely when and under what conditions a particular event needs to remain secret. Our framework is based on the temporal logic SecLTL, which combines the standard temporal operators of linear-time temporal logic with the modal Hide operator for the specification of information flow properties. We present a first monitoring algorithm for SecLTL specifications, based on a translation of SecLTL formulas to alternating automata, and identify open research questions and directions for future work.