This book is designed to critically review experimental findings on ionic polymers and colloidal particles and to prove a theoretical framework based on the Poisson-Boltzmann approach. Structure formation in ionic polymer solutions has attracted attention since the days of H. Staudinger and J. D. Bernal. An independent study on ionic colloidal dispersions with microscopy provided a compelling evidence of structure formation. Recent technical developments have made it possible to accumulate relevant information for both ionic polymers and colloidal particles in dilute systems. The outstanding phenomenon experimentally found is microscopic inhomogeneity in the solute distribution in macroscopically homogeneous systems.To account for the observation, the present authors have invoked the existence of the counterion-mediated attraction between similarly charged solute species, in addition to the widely accepted electrostatic repulsion.