Superabsorbent Polymers (SAP) added into high performance concrete prevents or reduces self-desiccation (or autogenous shrinkage) of concrete. The mechanism behind is free water release from saturated SAP leading to an increase of relative humility and promoting further hydration. Compared to plain cement paste, the addition of SAP changes the hydration process and the development of microstructure in concrete. In this chapter, the degree of hydration of cement in cement composites containing SAP is reviewed. The infl uence of SAP on the development of microstructure characteristics, i.e. porosity, pore size distribution, morphology and connectivity of bulk cement paste, interfacial transition zone between cement paste and SAP and the voids introduced by SAP is analyzed. It is concluded that the addition of SAP in the mixture increases the hydration degree of cement particle, this leads to a reduction of capillary porosity in the matrix. The additional of SAP meanwhile increase the void in the ITZ between SAP and matrix. Microcomputer tomography image shows that the voids introduced by SAP in the mixture are homogenously distributed.