We study the general structure of models for structure formation, with appl
ications to the reverse engineering of the model from observations. Through
a careful accounting of the degrees of freedom in covariant gravitational
instability theory, we show that the evolution of structure is completely s
pecified by the stress history of the dark sector. The study of smooth, ent
ropic, sonic. scalar anisotropic, vector anisotropic, and tensor anisotropi
c stresses reveals the origin, robustness, and uniqueness of specific model
phenomenology. We construct useful and illustrative analytic solutions tha
t cover cases with multiple species of differing equations of state relevan
t to thr current generation of models, especially those with effectively sm
ooth components. We present a simple case study of models with phenomenolog
ies similar to that of a Lambda CDM model to highlight reverse-engineering
issues. A critical-density universe dominated bl: a single type of dark mat
ter with the appropriate stress history can mimic a Lambda CDM model exactl
y. [S0556-2821(99)04208-3].