Thesearetheproceedingsofthe21stInternationalSymposiumonComputerand ? Information Sciences (ISCIS 2006) held in Istanbul, Turkey, November 1 - 3, 2006. ISCIS 2006 was organized by the Faculty of Engineering and Natural S- ences of Sabanc?University. These proceedings include updated versions of 106 papers selected by the program committee for presentation at the symposium. The increasing com- tition for presenting at the symposium, which relies on the success of previous ISCIS symposia,has led to many excellent papers and a higher standard overall. From a record-breaking 606 submissions, scheduling constraints dictated that only 106 papers could be accepted, resulting in a 17.5% acceptance rate. The program committee and external reviewers worked very hard and went through all submissions meticulously to select the best in a limited time frame. The selection process was especially hard when some of the good submissions could not be selected for publication due to space and time limitations. Another interesting fact about the submissions is that they were by authors from 41 countries covering all habitable continents (excluding Antarctica). This is a clear indication of the ever-increasing popularity of ISCIS symposia among researchersallaroundtheworldasamediumtoshareanddisseminatetheresults of their research activities.