The influence of moderator concentration in the eluent on retention in HPLC is reviewed. Problems with different moderators are discussed, special emphasis is brought to the omnipresent moderator water. The selectivity as a function of the stationary phase and the concentration of different moderators is demonstrated. The problems with gradient-elution and temperature-programming as a consequence of the always present water moderator in the eluent are discussed. Possible solutions to these problems are presented. The advantage of chemically bonded stationary phases in programmed analysis is compared with active supports.