A comparison is made between existing theories for self-assembling systems and experimental phase diagrams of complex systems made up of amphiphile/water/additives, with emphasis on phase transitions by changes in concentration. Evidence for different types of hexagonal (H) phases, in systems with detergent and lipid amphiphiles, are reviewed. It is shown that characteristics of the H phase and of the phase sequence (cubic phases with lipids and nematic phases with detergents) are revealed through the exponent defining the variation of the hexagonal parameter with amphiphile concentration. Emphasis is also given to results obtained in the ternary system sodium dodecyl (lauryl) sulfate / water / decanol, which exhibits the phase sequence isotropic (I) - H - nematic cylindrical (Nc) { [nematic biaxial (Nb)] - nematic discotic (Nd) - lamellar (L), with co-surfactant addition. Existing theories for self-assembling systems of rigid and exible rods predict the phase sequence I - (Nc) -H asa function of increased particle volume fraction upsilonp, with a triple point separating I - H and I - Nc - H phase sequences. Possible reasons for the non-trivial experimental I -H - Nc inversion in phase sequence are discussed. A complex path through the phase diagram is able to explain the experimental results in terms of changes in micellar growth and exibility induced by the co-surfactant. The role of the surfactant parameter, that expresses the curvature of the polar-apolar interfaces, explains much of the observed behavior, including the Nc - Nd transition with increase in decanol / amphiphile molar ratio.