In a coherent x-ray scattering experiment, interference of the waves diffracted across the sample is observed. This gives a speckle pattern in the observed scattering intensity, whereas a standard experiment leads to diffuse intensity or to broadening of a Bragg peak. Speckles correspond to the disorder configuration of the irradiated volume and their dynamics provide microscopic information of the time evolution of the sample microstructure. After a brief description of the techniques used for the observation of speckles in an x-ray experiment, some examples of measured time dynamics are given. These concern reversible fluctuations and irreversible decomposition of unmixing or ordering alloys. A few experiments in defect imaging are described and the studies opened by free electron lasers are briefly discussed. (C) 2011 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.