Loss of precision due to the conservative nature of compile-time dataflow analysis is a general problem and impacts a wide variety of optimizations. We propose a limited form of runtime dataflow analysis, called deferred dataflow analysis (DDFA), which attempts to sharpen dataflow results by using control-flow information that is available at runtime. The overheads of runtime analysis are minimized by performing the bulk of the analysis at compile-time and deferring only a summarized version of the dataflow problem to runtime. Caching and reusing of dataflow results reduces these overheads further. DDFA is an interprocedural framework and can handle arbitrary control structures including multi-way forks, recursion, separately compiled functions and higher-order functions. It is primarily targeted towards optimization of heavy-weight operations such as communication calls, where one can expect significant benefits from sharper dataflow analysis. We outline how DDFA can be used to optimize different kinds of heavy-weight operations such as bulk-prefetching on distributed systems and dynamic linking in mobile programs. We prove that DDFA is safe and that it yields better dataflow information than strictly compile-time dataflow analysis. (Also cross-referenced as UMIACS-TR-98-46)