Ad-Hoc, Mobile and Wireless Networks: 4th International Conference, ADHOC-NOW 2005, Cancun, Mexico, October 2005 Proceedings Violet R. Syrotiuk, Edgar Cha'vez (Eds.) From the Preface The 4th International Conference on Ad-Hoc Networks and Wireless (ADHOC-NOW 2005) was held in October 6-8, 2005 in Cancun, Mexico. Adhoc Now started as a workshop in 2002 and was held at the Fields Institute in Toronto. In 2003, it was held in Montreal… 2005 was the first year for the conference to move outside of Canada. The purpose of the conference is to create a collaborative forum between mathematicians, computer scientists, and engineers for research in the field of mobile ad hoc and sensor networks. In 2005, we received over 100 submissions from 22 different countries: Australia, Canada, china, France… Japan… Spain…- a true international conference. Of the papers submitted, we selected 27 for presentation at the conference and publication in the proceedings.