Soil Water Retention Curve, SWRC, saturated hydraulic conductivity, KS, and unsaturated hydraulic conductivity, K(h), are basic soil hydraulic functions and parameters. Here, h is the soil water pressure head (matrix potential). Knowledge of soil hydraulic functions and parameters is required for a successful formulation of principles leading to sustainable soil management, agricultural production and environmental protection. The basic soil hydraulic functions are strongly dependent upon the soil porous system. The systematic study on the relationships between soil hydraulic functions and the configuration of the soil porous system in individual soil taxons can be realized only if the soil hydraulic functions are fully physically interpreted. The physical parameters of soil hydraulic functions could be then related to topological characteristics of the soil porous system, or to realistic models of the soil porous system derived from the direct micromorphologic observations. The first step in proceeding in this direction is the formulation of the soil water retention curve SWRC as a function of the pore size distribution. Using this function, the unsaturated conductivity is defined physically, too, provided that the parameters of K(h) are physically definable, too. Our study was aimed at the derivation of soil hydraulic functions in bi-modal soils and their testing for measured data sets of real soils. We assumed and tested the existence of two domains of soil pores: The matrix (textural) and the structural (inter-aggregate) domain. The log-normal pore size distribution in both domains and the appropriate SWRC of both domains were formulated. The mutual relations of the domains were defined on the principal of superposition and the water retention curve of the whole soil was obtained. The pressure head hA at the minimum between two peaks of the pore size distribution function represents the boundary between the structural and matrix domains of the soil porous system. We tested the theory using experimental SWRC of several authors and the computed SWRC fitted well to the experimental ones. In addition to it, the role of structure and compaction was studied. The value of hA was in broad ranges between – 30 cm and about - 700 cm of pressure head, corresponding to equivalent radius r = 50 µm and r = 2.1 µm. The boundary between soil pore categories can not be taken as a fixed value for all soils and all types of soil use. If the structure is destroyed, the bi-modal system approaches the mono-modal system. Soil water retention curves are significantly changed due to compression in soils of low aggregate stability. The change is substantially smaller in soils of high aggregate stability. The pore size distribution is changed substantially in both, the structural and matrix domains of soils characterized by low aggregate stability. The change in the matrix domain is relatively small in soils with well developed structure and high aggregate stability, while the change is more expressed in the structural domain. The decrease of structural porosity is more pronounced when compared to decrease of the total porosity due to compression. It indicates that the value of structural porosity is mainly influenced by compression. The matrix porosity is in majority of instances increasing with the increase of compression The explicit form of the relative conductivity function was formulated for matrix domain and for the structural domain of pores. Two or alternatively three parameters relating the model of porous system to the reality of porous systems were obtained by optimization for the experimental data of soils. The values of parameters in one domain differ substantially from the parameters of the other domain. We conclude that the porous systems in the two domains differ and that they have to be treated separately when their hydraulic functions are studied and further applied in the solution of transport phenomena in real soils. The physical interpretation of parameters is questionable and it has to be checked by quantification of micromorphologic observation of the soil porous system in the future. The assumption on the direct description of tortuosity and pores connectivity by the parameters is not fully supported by our data and the relations look as more complicated. Acknowledgment: This study was funded through the Grant Agency of the Czech Republic, GACR 103/05/2143.