This critical review provides an overview on the formation of pi-stacks of functional dyes in solution, aiming to acquaint young researchers with this topical research field and to stimulate further advance in supramolecular dye chemistry. Different mathematical models that have been proposed and applied for the description of aggregation equilibria of pi-systems in solution are discussed. The factors that have significant impact on the structural features of aggregates and the thermodynamics of pi-pi stacking such as electrostatic interactions, size and geometry of the dye molecules are covered in this review. A comparison of the binding strength is made for different classes of functional pi-conjugated systems, from simple benzene to more extended polycyclic hydrocarbon molecules, including triphenylenes and hexabenzocoronenes, heteroaromatic porphyrins and phthalocyanines, quadrupolar naphthalene and perylene bisimides, dipolar or even zwitterionic merocyanines and squaraines, and some macrocyclic dyes. Solvent effects on binding constants are analysed by linear free energy relationships with various solvent polarity scales (98 references with multiple entries).