The preparation of silica membranes by chemical vapor deposition (CVD) is critically reviewed, with focus placed on the two most common supports used, Vycor glass and porous alumina. The major application of the membranes is hydrogen separation, but other gases can also be separated by functionalization of the silica. A description is given of the different gas transport mechanisms, and a comparison is provided to palladium membranes. •Silica-based membranes for hydrogen separation are critically reviewed.•Chemical vapor deposition (CVD) on Vycor glass and alumina supports is described.•The main CVD geometries, “one-sided” and “opposing reactants” feed are discussed.•Different operative gas transport mechanisms are presented.•The use of silicon and non-silicon CVD precursor compounds is covered. Hydrogen separation membranes are important in the gas separation field and among these, silica-based membranes have emerged as promising materials at high temperatures due to their high permeation rates, high selectivity, hydrothermal stability, resistance to poisons, and mechanical strength. A critical review of the preparation of silica membranes by chemical vapor deposition (CVD) is given, with special attention placed on the two major supports used, Vycor glass and porous alumina. The coverage includes the different gas transport mechanisms that occur through silica membranes, which are convective flow, Knudsen diffusion, molecular sieving, activated diffusion, and solid-state diffusion. A description is made of the two main CVD geometries, which are the “one-sided” feed and “opposing reactants” feed configurations. The results of numerous studies in which CVD precursors such as silicon alkoxides and organosilicon species, as well as non-silicon element compounds are used to control pore size and surface properties are presented. Also the effect of varying reaction conditions such as temperature, pressure, and reaction time are compared. Finally, the permeation properties of the silica membranes are compared to those of palladium membranes, both types of membranes are employed for hydrogen separation in membrane reactors, and the advantages of silica membranes in these reactor systems are discussed.