Introduction In this memoir we aim at presenting, in a unified way, various and intercon- nected basic aspects of the regularity theory of solutions to general, non-linear second order parabolic systems with polynomial p-growth p ≥ 2 . Specifically, we are dealing with systems of the type (0.1) ut − div a(x, t, u, Du) = 0 and related non-homogeneous ones ut − div a(x, t, u, Du) + H = 0 , H ∈ D (ΩT ) , under natural p-growth and ellipticity assumptions on the vector field a: ΩT × RN × RNn → RNn that is (0.2) |a(x, t, u, w)| ≤ L(1 + |w|2) p−1 2 holds for every (x, t) ∈ ΩT , u ∈ RN and w ∈ RNn. The system in question is considered in the cylindrical domain ΩT = Ω × (−T, 0) , where Ω denotes a bounded domain in Rn, n ≥ 2, and T 0 in the following Du will always denote the gradient with respect tot he space variables x, i.e. Du(x, t) ≡ Dxu(x, t). The precise assumptions will be every time specified when presenting the various theorems obtained in the paper such assumptions will be in most of the cases natural and optimal with respect to the implied conclusions. The notion of (weak) solution adopted here, and in the rest of the paper, prescribes of course that a map u ∈ C0(−T, 0 L2(Ω, RN)) ∩ Lp(−T, 0 W 1,p (Ω, RN)) is a (weak) solution to (0.1), under the assumption (0.2) for p ≥ 2, iff ΩT u · ϕt − a(x, t, u, Du),Dϕ dx dt = H, ϕ holds for everyϕ ∈ C0 ∞ (ΩT , RN). We shall provide three basic types of interconnected regularity results for so- lutions to (0.1), and in fact one of our aims is also to give a unifying approach to different regularity issues, emphasizing the interactions between the various techni- cal aspects of the problems treated. We shall give optimal partial regularity results for solutions, the first results on the Hausdorff dimension size of the singular sets ix

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