Computations of long-time hygrothermal effects in concrete structures necessitate a physically based model for autogenous shrinkage and swelling of hardened portland cement paste. The present goal is to propose such a model. As known since 1887, the volume of cement hydration products is slightly smaller than the original volume of cement and water. However, this does not mean that the hydration reaction causes contraction of the cement paste and concrete. According to the authors’ recently proposed paradigm, the opposite is true for porous cement paste as a whole. The growth of C-S-H shells around anhydrous cement grains pushes the neighbors apart and thus causes volume expansion of the porous cement paste as a whole, while the nanoscale volume contraction of hydration products contributes to porosity. The growth of ettringite and portlandite crystals may also cause additional expansion. On the material scale, the expansion always dominates over the contraction, i.e., the hydration per se is, in the bulk, always expansive, while the source of all of the observed shrinkage, whether autogenous or due to external drying, is the compressive elastic or viscoelastic strain in the solid caused by a decrease of chemical potential of pore water, with the corresponding decrease in pore humidity, increase of solid surface tension and, mainly, decrease of disjoining pressure. The low density C-S-H and high density C-S-H are distinguished in the proposed model. The selfdesiccation, shrinkage and swelling can all be predicted from one and the same unified model, as confirmed by comparisons with with the existing experimental evidence. The model is ready for use in finite element programs.