Cardiolipin is a phospholipid with negatively charged headgroups. This lipid is structurally unique in its quadruple-chained configuration and functionally unique in its nearly exclusive involvement in the cellular energy production processes. Whether the structural uniqueness leads to the functional exceptionality has long been an open question. Like other phospholipids, cardiolipin is a liquid crystal and demonstrates polymorphism when it is purified and mixed with water. Many studies have been dedicated to examining the phase behavior of cardiolipin liquid crystals in an effort to understand the driving forces behind phase transitions and to ultimately decipher the structure-function relation. However, few, if any, studies have thus far systematically investigated cardiolipin phase behavior in broad temperature and concentration ranges. In this thesis, small- and wide-angle X-ray scattering techniques were employed to study the phase behavior of cardiolipin-water mixtures. A phase diagram was mapped in lipid concentrations from 32.9 wt% to 85.4 wt% and temperatures from -20 °C to 60 °C. Two striking features were observed in this cardiolipin phase diagram: the presence of a lamellar-lamellar phase separation region and a phase displaying crystalline-like X-ray scattering patterns. Based on the X-ray scattering data underlying the phase diagram, electron density maps of the cardiolipin liquid crystals were reconstructed with two different methods and their structural parameters were derived, both within and across a phase boundary. A relationship between phase behavior and structure was observed from this structural information and utilized to construct a "structure map". Based on the structure map, an energetics view was provided to explain the observed phase transitions and to explore the nature and origins of the two striking phase behaviors.