The ability of an Independent Protection Layer (IPL) to achieve a given level of risk reduction is dependent upon its fulfillment of several core attributes. A key provision for any IPL to be considered effective and functionally adequate is its capability to respond to a process demand quickly enough to stop the propagation of the hazard scenario it was designed to prevent. While this seems obvious and reasonable, the estimation of Process Safety Time and the specification of IPL Response Times are more complex, and often deferred or overlooked altogether. What is Process Safety Time? How is it determined? When? And by whom? This article examines the relationship between Process Safety Time and IPL Response Times, essential variables for the justification of IPL effectiveness, and their impacts on the success of Layer of Protection Analysis. (C) 2015 American Institute of Chemical Engineers