A quasistatic theory of continuous materials with microstructure is formulated in a highly mathematical fashion. The body is defined as a lattice and a microstructure is specified by assigning a finite number of order parameters which are interpreted as the coordinates, in a local chart, of a point of a differentiable manifold which is embedded in an Euclidean space of higher dimension. Interactions are described by particular mappings from pairs of subsets to normed vector spaces. These interactions are called quasi-balanced if they are expressed as integrals of certain density function with respect to volume measure. Several classical theorems of continuum mechanics and boundary conditions are treated in detail. Working is defined and equations of motions are derived by assuming that the global working is frame-indifferent. The physical situation is recovered by imposing the virtual microstructure to remain on the manifold depicted by order parameters.