The aggregation behavior of asphaltenes in apolar solvents is studied using a molecular thermodynamic approach. The theory is based on a molecular model for asphaltene aggregates that describes them as aromatic cores, composed of stacked aromatic sheets, surrounded by aliphatic chains. Using this simple molecular model, an analytical expression is developed for the free energy of aggregation that incorporates five contributions due to (1) transfer of the polyaromatic rings from the solvent into the aromatic core, (2) mixing of the aliphatic chains with the solvent, (3) deformation of the aliphatic chains, (4) steric repulsion among the aliphatic chains, and (5) aggregate core -solvent interactions. The proposed approach provides a qualitative description of the main experimental trends observed for asphaltene aggregation. Specifically, the experimentally observed variation of eme values and aggregate size with (1) asphaltene molecular characteristics, (2) asphaltene concentration, (3) solvent composition, and (4) temperature has been qualitatively reproduced by the theory. In addition, the thermodynamic molecular model developed does not utilize any information derived from experiments on asphaltene solutions, and therefore, it is strictly predictive.