We present a general method to extend results on Hilbert space operators to the Banach space setting by representing certain sets of Banach space operators Γ on a Hilbert space. Our assumption on Γ is expressed in terms of α-boundedness for a Euclidean structure α on the underlying Banach space X. This notion is originally motivated by R-or γ-boundedness of sets of operators, but for example any operator ideal from the Euclidean space ℓ 2 n to X (like the γradonifying or the 2-summing operator ideal) defines such a structure. Therefore our method is quite general and flexible and allows to unify the approach to seemingly unrelated theorems. Conversely we show that Γ has to be α-bounded for some Euclidean structure α for it to be representable on a Hilbert space.By choosing the Euclidean structure α accordingly we get a unified and more general approach to classical factorization theorems like the Kwapień-Maurey factorization theorem, an improved version of the Banach function space-valued extension theorem of Rubio de Francia, a quantitative proof of the boundedness of the lattice Hardy-Littlewood maximal operator and the equivalence of the UMD and the dyadic UMD + property on Banach function spaces. Furthermore we use these Euclidean structures to build vector-valued function spaces, which enjoy the nice property that any bounded operator on L 2 extends to a bounded operator on these vector-valued function spaces, which is in stark contrast to the extension problem for Bochner spaces. With these spaces we define an interpolation method, which has formulations modelled after both the real and the complex interpolation method.Using our representation theorem we prove a quite general transference principle for sectorial operators on a Banach space, enabling us to extend Hilbert space results for sectorial operators to the Banach space setting. We extend and refine the known theory based on R-boundedness for the joint and operator-valued H ∞ -calculus and the "sum of operators" theorem for commuting sectorial operators. Moreover we extend the classical characterization of the boundedness of the H ∞ -calculus on Hilbert spaces in terms of BIP, square functions and dilations to the Banach space setting. Furthermore we establish via the H ∞ -calculus a version of Littlewood-Paley theory and associated spaces of fractional smoothness for a rather large class of sectorial operators. Our abstract setup allows us to reduce assumptions on the Banach space geometry of X, such as (co)type and UMD. We conclude with some sophisticated counterexamples for sectorial operators, with as a highlight the construction of a sectorial operator of angle 0 on a closed subspace of L p for 1 < p < ∞ with a bounded H ∞ -calculus with optimal angle ωH∞ (A) > 0.