I've heard (and understand) all of the benefits of using async data calls, but I have never heard if there are any negatives to setting the async page directive to true... I have a large (mostly legacy) asp.net webforms site running under 4.5 and make several calls out of my business tier that would benefit from using async. However, the pages that ultimately call into those business methods don't know that they might call async processes. So, that leads me to the question, Is there some reason why I should not run a page with the async directive set to true? Would there be a drawback to setting it to true for the whole site/application?