Progress in understanding hydration is summarized. Evidence supports the geochemistry dissolution theory as an explanation for the induction period, in preference to the inhibiting layer theory. The growth of C-S-H is the principal factor controlling the main heat evolution peak. Electron microscopy indicates that C-S-H "needles" grow from the surface of grains. At the peak, the surface is covered, but deceleration cannot be attributed to diffusion control. The shoulder peak comes from renewed reaction of C(3)A after depletion of sulfate in solution, but release of sulfate absorbed on C-S-H means that ettringite continues to form. After several days space becomes the major factor controlling hydration. The use of new analytical technique is improving our knowledge of the action of superplasticizers and leading to the design of molecules for different applications. Atomistic modeling is becoming a topic of increasing interest. Recent publications in this area are reviewed. (C) 2015 Elsevier Ltd. All rights reserved.