The invention provides a foam control formulation in the form of a microemulsion, the formulation comprising: (a) from 5 to 70% w/w of primary surfactant, this surfactant having an HLB of from 1 to 12 and/or a cloud point of from 20 to 70° C.; (b) from 2 to 40% w/w of water-insoluble organic carrier liquid; and (c) water. Also provided is the use of this formulation to prevent and/or reduce foam in a fluid system, or as a processing aid to control foam production in a fluid system. The formulation may be used in an aqueous fluid system, such as an oilfield.