I. Structure and identification of mycolic acids 143 A. Generalized structures and properties of mycolic acids 143 1. An historical perspective on mycolic acid structure 143 2. General motifs found in mycolic acids 144 3. Preparation and identification of mycolic acids 145 B. Structural variation in nonmycobacterial mycolic acids 146 C. Structural variation in mycobacterial mycolic acids 148 1. Chemotaxonomy of mycobacterial species 148 2. Class variation between species 149 3. Microheterogeneity 149 II. Biosynthesis of mycolic acids 152 A. Mero and alpha branch synthesis 152 1. Short-chain lipid synthesis: the alpha branch and mero primers 152 2. Long-chain lipid synthesis: the mero±INH connection 154 B. Meromycolate modification reactions 156 1. The desaturation 156 2. Modifications involving methyl transfer from SAM 157 3. Oxidative modifications of the meromycolate 160 C. The condensation and carriers 161 D. Cell-free``complete mycolate''synthesizing systems 163 III. Physiological function of mycolic acids 164 A. General features and functions of the cell wall of mycolic acid containing bacteria 164 B. Structure of the mycobacterial cell wall 165 1. Electron microscopic examinations 165 2. Model of the mycobacterial cell wall 167 3. Biophysical examinations of cell wall structure 167 C. Physiological roles of structural variation in cell-wall bound mycolic acids 168 1. Generic properties of mycolic acids 168 2. Cis/trans olefins and thermal adaptation 169 3. Trans cyclopropanation 170 4. Cis cyclopropanation 170 5. Oxygenated mycolates 171 D. Physiological roles of non-cell wall bound mycolic acids 172 1. The role of mycolic acid in the immune response to mycobacterial infection 172 2. The role of mycolic acids in the pathogenesis of mycobacterial disease 172 3. Adjuvant activity and immunogenicity 173