In this paper, the Chlorophyll a aggregation behavior is investigated in relation to the nature of the organic solvent, the composition of aqueous mixtures with different organic solvents, and the pigment concentration. Because of the complexity of the system under investigation and the number of parameters that can influence the Chlorophyll a aggregation processes, the data relative to the physicochemical characterization of the species will be discussed in three sessions, corresponding to different ranges of the solvent composition. Chlorophyll a behavior is discussed when present mainly as a monomer, as a polymer or as a "micellar" species, respectively. Particular attention is devoted to the pigment response in the water rich region, where the solvent behaves almost as pure water and the aggregation processes are essentially driven by hydrophobic interactions. The overall data above-reported are consistent with the formation of spherical-shaped aggregates of Chlorophyll a (of pheophytin a) molecules in which the phytil chains are segregated in the inner part, thus fostering the pigments to expose the macrocyclic heads toward the bulk water solvent. Besides optical techniques (UV-vis, fluorescence, and circular dichroism) used to investigate the spectral properties of the chlorophyll, the aggregation state is investigated by resonance and dynamic light scattering, nuclear magnetic resonance self-diffusion, transmission electron microscopy, and adsorption-stripping cyclic voltammetry.