This paper reviews the current state of the art of requirements engineering (RE) research and identifies RE research challenges for future systems. First, the paper overviews the highlights of RE research over the past two decades; the research is considered with respect to requirements technologie, including notations and methodologies, developed to address specific RE tasks, such as elicitation, modeling, and analysis. Such a review enables us to identify mature areas of research, as well as areas that warrant further investigation. Next, we identify several research challenges posed by emerging systems for the future. In order to help delineate the scope of future RE research directions, we then identify several strategies for performing RE research. (The spectrum of research strategies ranges from empirical research to paradigm shifts.) Finally, within the context of these RE research strategies, we identify "hot areas" of research that address RE needs for emerging systems of the future.