I Introduction to Chemical and Physical Gels.- Covalent Macromolecular Gels.- Physical Gels and Biopolymers (in French).- Gelation of Physical Gels: The Gelatin Gels.- Clay Minerals: A Molecular Approach to Their (Fractal) Microstructure.- Structure of Random Materials.- II Microemulsions.- Ionic Microemulsions.- The Sturcture of Microemulsions.- Random Bicontinuous Microemulsion Predictions of Talmon-Prager and de Gennes Models. Structure Experiments and Determination of the Flexibility of the Interface.- III Packings.- The Relation Between the Structure of Packings of Particles and Their Properties.- Dense Packings of Hard Grains: Effects of Grain Size Distribution.- IV Percolation.- Shapes, Surfaces, and Interfaces in Percolation Clusters.- Numerical Simulation of Percolation and Other Gelation Models.- Formation of Polymer Networks: Treatment of Stochastic and Spatial Correlations Using the Mean-Field Approximation.- Dynamical Processes in Random Media.- Flicker (1/f) Noise in Percolation Networks.- V Properties of Branched Polymers and Gels.- Structure of Branched Polymers.- Size Distribution and Conformation of Branched Molecules Prepared by Cross-linking of Polymer Solutions.- Viscoelastic Effects Occurring During the Sol-Gel Transition.- Neutron Scattering Investigation of the Loss of Affineness in Deswollen Gels.- Scaling NMR Properties in Polymeric Gels.- VI Elastic and Dielectric Properties of ill-connected Media.- Scaling and Crossover Considerations in the Mechanical Properties of Amorphous Materials.- Elastic Properties of Depleted Networks and Continua.- The Elastic Properties of Fractal Structures.- Elasticity of Percolative Systems.- Electrical Properties of Percolation Clusters: Exact Results on a Deterministic Fractal.- to the Ionic Transport in Semi-Crystalline Complexes.- Williams-Watts or Cole-Cole: The Universal Response of Polymers.- VII Aggregation and Instability.- Random Kinetic Aggregation.- Scaling of Cluster Aggregation.- Dynamic Scaling in Aggregation Phenomena.- Fractal Growth of Viscous Fingers: New Experiments and Models.- Fingering Patterns in Hele-Shaw Flow.- VIII Flow and Diffusion in Porous Media.- Two-Component Transport Properties in Heterogeneous Porous Media.- Multiphase Flow in Porous Media.- Capillary and Viscous Fingering in an Etched Network.- Acoustics and Hydrodynamic Flows in Porous Media.- Dynamics of Saturated and Deformable Porous Media.- The Art of Walking on Fractal Spaces and Random Media.- Flow in Porous Media and Residence Time Distribution (in French).- IX Transitions.- Wetting of a Random Surface: Statistics of the Contact Line.- Wettability of Solid Surfaces: A Phenomenon Where Adsorption Plays a Major Role.- Binary Liquid Gels.- Phase Separation and Metastability in Binary Liquid Mixtures in Gels and Porous Media.- Index of Contributors.

The Second Winter School on the "~hysics of Finely Divided Matter" was held at the Centre de Physique des Houches from 25 March to 5 April 1985. This meeting brought together experts from the areas of gels and porous media. People with different backgrounds - chemists, physicists - from university as well as industrial labora­ tories, had the opportunity to compare their most recent experimental and theoreti­ cal results. Although the experimental situations and techniques may seem at first sight unrelated, the theoretical interpretations are very similar and may be divided roughly into two categories: percolation and aggregation. These are present for the description of the synthesis of some gels as well as for a description of the struc­ ture of packings. They are also a precious help for understanding flows in porous media and hydrodynamic instabilities such as viscous fingering. A different aspect, still in its early stages, deals with the influence of a ran­ dom medium on a phase transition. This leads to metastable states and is interpreted in terms of random fields. The following topics were covered: introduction to physical and chemical gels structure of packings and porous media microemulsions percol ati on aggregation elastic and dielectric properties of ill-connected media properties of gels near and far from the gelation threshold flow, diffusion and dispersion in porous media transitions in porous media. Most of these are rapidly growing subjects, and we hope that these proceedings will serve as a reference for those entering this fascinating area.