This article reviews our understanding of ionization processes of weak polyelectrolytes. The emphasis is put on a general introduction to site binding models, which are able to account for many experimental features of linear and branched polyelectrolytes, including dendrimers. These models are fully compatible with the classical description of acid–base equilibria. The review further discusses the nature of the site–site interaction and role of conformational equilibria. Experimental charging data of numerous weak polyelectrolytes are discussed in terms of these models in detail.