We have divided the book into four parts. To obtain an idea of the theoretical basis of gradient elution separations, it is necessary to become acquainted with the fundamentals of the phenomena that control the influence of the mobile phase and its composition on retention behaviour in various liquid chromato-graphic systems under isocratic conditions. Therefore, it was felt necessary to devote one part to this topic. In this first part, several models are discussed that allow descriptions of the interrelationships between the composition of the mobile phase and retention, as characterized by capacity factors, in chromatography on polar adsorbents, on polar bonded phases and on ion exchangers, in reversed-phase systems and in ion-pair chromatography. The quantitative description of these relationships is essential for developing any theory of qradient elution liouid chromatoaraphy.