Expresso is an experiment management system that is designed to assist biologists in planning, executing, and interpreting microarray experiments. It serves as a unifying framework to study data-driven application composition systems, as envisaged under the NSF Next Generation Software (NGS) program. Physical and analytical stages of the microarray process are mirrored in Expresso with computational models from biophysics, molecular biology, biochemistry, robotics, image processing, statistics, and knowledge representation. These models are pushed deeper (earlier) into the design process to help avoid costly design errors and to provide, as needed, surrogate functions for the traditional stages of microarray experiments. In this paper, we describe ongoing work in the design of Expresso, with specific reference to application composition, application optimization, experiment protocol design, and ?closing the loop.?