Phospholipid bilayer membranes undergo structural changes called phase transitions in response to various external environmental factors. They exhibit markedly complicated phase behavior due to the nonhomogeneous structures. We have investigated the bilayer phase transitions of various phospholipids with different molecular structures to elucidate the environmental adaptation of biological membranes. The constructed thermodynamic phase diagrams of the phospholipid bilayer membranes have revealed the correlation between the lipid molecular structures and the thermotropic and barotropic bilayer phase behavior. In this review, several types of bilayer phase behavior of representative glycerophospholipids contained in biological membranes, saturated diacyl-posphatidylcholines (PCs), - phosphatidylethanolamines (PEs) and -phosphatidylglycerols (PGs), are explained, and subsequently our lipid membrane studies based on the bilayer phase diagrams, formation of nonbilayer phases (chain interdigitation for saturated diacyl- PCs and inverted hexagonal transformation for unsaturated diacyl-PEs) and fluorescence imaging of bilayer packing for saturated diacyl-PCs, are introduced.