The paper is devoted to a comprehensive study of smoothness of inertial manifolds for abstract semilinear parabolic problems. It is well known that in general we cannot expect more than C 1,ε -regularity for such manifolds (for some positive, but small ε). Nevertheless, as shown in the paper, under the natural assumptions, the obstacles to the existence of a C n -smooth inertial manifold (where n ∈ N is any given number) can be removed by increasing the dimension and by modifying properly the nonlinearity outside of the global attractor (or even outside the C 1,ε -smooth IM of a minimal dimension). The proof is strongly based on the Whitney extension theorem. Contents 1. Introduction 1 2. Preliminaries I: Taylor expansions and Whitney Extension Theorem 6 3. Preliminaries II: Spectral gaps and the construction of an inertial manifold 10 4. Main result 18 5. Verifying the compatibility conditions 25 6. Examples and concluding remarks 30 References 34