The development of a general theory of clustering is sketched. The pri
mary focus is on the use of the pair connectedness function in problem
s of correlated clustering and on the Ornstein-Zernike formalism that
has been developed to evaluate this function and relate it to observed
mesoscopic and macroscopic properties. The treatment stresses applica
tions and potential applications that are directly relevant to colloid
problems, especially percolation in microemulsions and gelation in po
lymer solutions.