Hofmeister ion series is ubiquitous and turns up frequently in many chemical and biological processes. The explanations for its mechanism of action on target macromolecules in aqueous solutions fall into two categories. The first one states that ions affect the behavior of macromolecular solutes in the aqueous solution by interacting with water, while the other one claims that direct interactions between ions and macromolecules are the key point for comprehending the Hofmeister ion series. In this paper, research works on the two mechanisms of action are elaborately reviewed. In fact, actual solution systems are inevitably complicated by rather delicate balance of all the interactions (e.g., water-water, water-ion, water-solute, ion-solute, cation-anion) and the above mentioned two viewpoints explain the Hofmeister ion series just from one side. More work is still needed to dig into the secret behind the Hofmeister ion series.

Contents
1 Introduction
2 Indirect mechanism of action
3 Direct mechanism of action
4 Conclusion and outlook