Recently there has been increasing interest in the development of progress indicators for SQL queries. In this thesis we present a lightweight online framework for this problem. Our framework is online, in the sense that it refines its estimate of query progress based on feedback received during query execution. It is lightweight, since our techniques are designed to impose minimal overhead on query execution without sacrificing accuracy of estimates. Our framework can estimate progressively the output size of various relational operators and pipelines. These include binary and multiway joins as well as typical grouping operations and combinations thereof. We describe the various algorithms used to efficiently implement the estimators and present the results of a thorough evaluation of a prototype implementation of our framework in an open source data manager. Our results demonstrate the feasibility and practical utility of the approach presented herein.