Bacteria precisely remodel the fluidity of their membrane bilayer via the incorporation of proportionally more unsaturated fatty acids (or fatty acids with analogous properties, such as branched-chain fatty acids) as growth temperature decreases. This process, termed homeoviscous adaptation, is suited to disrupt the order of the lipid bilayer and optimizes the performance of a large array of cellular physiological processes at the new temperature. As such, microbes have developed molecular strategies to sense changes in membrane fluidity, provoked by a decrease in environmental temperature, and initiate cellular responses that upregulate the biosynthesis of either unsaturated, terminally branched, or shorter chain fatty acids. In this review, we describe some of the basic molecular strategies that bacteria use to sense temperature. While the activities of all biomolecules are altered as a function of temperature, the thermosensors we focus on here are molecules whose temperature sensitivity provides information about the thermal environment that is used to trigger an appropriate adjustment of membrane architecture. We also discuss selected examples of membrane and lipopolysaccharide remodeling induced by cold, which involves changes in the activity of fatty acid biosynthetic enzymes or the expression of acyltransferases that modify the lipid A.