In this review we are primarily concerned with the use of apparent molal volume and apparent molal compressibility of polyelectrolytes along with their corresponding monomers in elucidating the ion-ion and ion-solvent interactions in aqueous and nonaqueous solutions. Though the main emphasis will be given to synthetic polyelectrolytes, the biochemical significant (natural) ones are not excluded altogether. It is hoped that this information will provide researchers with up-to-date knowledge of the subject matter and give a new impetus to further study on the subject. Most of the works on molal volumes and ultrasonic velocity measurements were confined up to the middle sixties to solutions of simple electrolytes. These works gave valuable information about the structure of solutions as well as about the fundamental properties of solutions.