Correlation functions are sensitive to the presence of a boundary. Surface modulations give rise to modified near surface correlations, which can be measured by scattering probes. To determine these correlations, we develop a perturbative calculation in deformations in height from a flat surface. The results, combined with a renormalization group around four dimensions, are also used to predict critical behavior near a self-affinely rough surface. We find that a large enough roughness exponent can modify surface critical behavior.PACS numbers: 68.35. Rh, 68.35.Ct, 05.70.Jk, 64.60.Fr Bulk properties, such as magnetization, as well as correlation functions are modified on approaching a surface. In particular, critical behavior near surfaces or defects, which is quite different from the bulk, has been extensively studied theoretically [1][2][3][4], by experiments [5][6][7], and in simulations [8]. Along with this development, the method of grazing incidence of x-rays and neutrons [3] has become a standard tool of probing critical behavior near surfaces and interfaces [5][6][7].Most theoretical studies have been restricted to flat surfaces. However, real surfaces mostly deviate from this idealized picture. Possible deviations can be divided into two classes: (i) advanced experimental methods, e.g., lithographic preparation, allow one to endow surfaces with specific, regular geometrical patterns down to the nanometer scale, with important applications in technology and material science [9]; (ii) surfaces or interfaces can be naturally rough, e.g., due to growth, fracture, or erosion. Most common are self-affinely rough surfaces, in which the root mean square height fluctuations on a length scale L grow as L ζ , where ζ < 1 is the so-called roughness exponent. Self-affine scaling is predicted by many numerical and analytical models of surface growth [10], and is also observed in a number of experiments [11].In this work we show that the shape of the surface has a distinct influence on the properties of an adjacent medium with long-range correlations. This is demonstrated for two-point correlation functions near a critical point of the medium, for both cases (i) and (ii) outlined above. The diffuse scattering of x-rays and neutrons at grazing incidence due to such correlations appears in addition to what would have been observed if the surface was separating two homogeneous media [12]. The modified correlations may thus provide an additional and indirect means of characterizing the surface profile. This may be of value when other techniques are not possible, as in the case of the interior surface of a glass, or an internal crack, whereas scattering from a critical fluid or binary alloy coating the surface may be feasible.In order to study the effects of the surface shape, we develop a perturbative expansion of two-point correlation functions in the deformations of the height profile. Initially for a Gaussian field, the calculations are carried out to second order. Already at the first order, the two-point correlatio...