It is reported on the structural polymorphism of the main amphiphilic cell membrane compounds, phospholipids and glycolipids, with special regard to calorimetric analyses. These lipids may form a large variety of aggregate structures in dependence on their chemical primary structure, on temperature, water content, and concentrations of cations. The entity of aggregate structures is usually called the phase diagram of the respective lipids. This should, however, not to be confused with the gel and liquid crystalline phases of the hydrocarbon chains of lipids, which differ in the fluidity of the acyl chains, and between which a first order transition can be observed. Thus, in this contribution, exemplary results are presented on the structural behaviour of biologically important lipids including their phase behaviour and structural preferences under different ambient conditions, and how phase and structural transitions are connected with enthalpy changes. (c) 2005 Elsevier B.V. All rights reserved.