We study the general structure of models for structure formation, withapplications to the reverse engineering of the model from observations.Through a careful accounting of the degrees of freedom in covariantgravitational instability theory, we show that the evolution ofstructure is completely specified by the stress history of the darksector. The study of smooth, entropic, sonic, scalar anisotropic, vectoranisotropic, and tensor anisotropic stresses reveals the origin,robustness, and uniqueness of specific model phenomenology. We constructuseful and illustrative analytic solutions that cover cases withmultiple species of differing equations of state relevant to the currentgeneration of models, especially those with effectively smoothcomponents. We present a simple case study of models withphenomenologies similar to that of a ΛCDM model to highlightreverse-engineering issues. A critical-density universe dominated by asingle type of dark matter with the appropriate stress history can mimica ΛCDM model exactly.