DREAM is a Dialogue for Reverse Engineering Assessments and Methods. The main objective is to catalyze the interaction between experiment and theory in the area of cellular network inference and quantitative model building in systems biology. The fundamental questions for DREAM are simple: How can we assess how well we are describing the networks of interacting molecules that underlie biological systems? and How can we know how well we are predicting the outcome of previously unseen experiments from our models? The answer to these questions are not so simple. Researchers have used a variety of algorithms to deduce the structure of biological networks and/or to predict the outcome of perturbations to their systems. They have also eveluated the success success of their methodologies using diverse non-standardized metrics. However what is still needed, and what DREAM aims to achieve, is a fair comparison of the strengths and weaknesses of the methods and a clear sense of the reliability of the models that researchers create.