The ISAAC Group in Pseudo-di?erential Operators (IGPDO) was formed at the Fourth ISAAC Congress held at York University in Toronto in 2003 and the ?rst volume entitled Advances in Pseudo-di?erential Operators and devoted to papers focussing on pseudo-di?erential operators and its diverse applications was then initiated and published in Professor Israel Gohberg’s series Operator Theory: - vances and Applications in 2004. As a satellite conference to the Fourth Congress of European Mathematics held at Stockholm University in 2004,the International ConferenceonPseudo-di?erentialOperatorsandRelatedTopicswasheldatVaxj ¨ o ¨ University in Sweden. Prompted by the enthusiasm of the participants, the second volume with similar scope and entitled Pseudo-di?erential Operators and Related Topics was published in the same series in 2006. Members of IGPDO met again at the Fifth ISAAC Congress held at Univ- sit` a di Catania in Italy in July 2005. Core members of the group encouraged the publication of a sequel to the Toronto Volume and the Vaxj ¨ o ¨ Volume. The vision is to seek new directionsfor the broadsubjectonpseudo-di?erentialoperatorsand the strategy is to devote the Catania Volume not only to papers based on lectures given at the special session on pseudo-di?erential operators, but also invited - pers that bear on the themes of IGPDO. In order to re?ect the goal and vision of IGPDO, the Catania Volume is entitled Modern Trends in Pseudo-di?erential Operators.