The existence of a topological double-covering for the GL(n, R) and diffeomorphism groups is reviewed. These groups do not have finite-dimensional faithful representations. An explicit construction and the classification of all SL(n, R), n = 3, 4 unitary irreducible representations is presented. Infinite-component spinorial and tensorial SL(4, R) fields, "manifields", are introduced. Particle content of the ladder manifields, as given by the SL(3, R) "little" group is determined. The manifields are lifted to the corresponding world spinorial and tensorial manifields by making use of generalized infinite-component frame fields. World manifields transform w.r.t. corresponding Dif f (4, R) representations, that are constructed explicitly.