This special issue of the Journal of Statistical Theory and Practice is dedicated to the memory of Professor Theophilos Cacoullos, who passed away on April 3, 2020. He was born on April 5, 1932, in Cyprus. He received his M.A. in 1960 and his Ph.D. in 1962 in Mathematical Statistics from Columbia University, New York, under the supervision of Allan Birnbaum and T. W. Anderson, respectively. In 1961–62, he worked as a Research Associate at Stanford University. In Sequoia Hall, he developed his ties with Herman Chernoff, Ingram Olkin, Charles Stein, and Samuel Karlin, among others. He was employed in several Universities in the USA, before returning in 1968 to the National and Kapodistrian University of Athens to begin his 31-year career in the department of Mathematics. Cacoullos visited McGill University, Canada, in 1972–73 and 1975–76, always enjoying the collaboration and friendship of Harold Ruben. Later in the fall of 1983, at the invitation of Herman Chernoff, he delivered a series of special lectures at the Statistics Center of MIT. In the spring of 1989, he visited the Center for Multivariate Analysis under C. R. Rao, then at the University of Pittsburgh.

Cacoullos published over 70 papers in international journals, and lately he became quite interested in the nature and history of Probability and Statistics. Also, he wrote seven textbooks on Probability and Statistics in Greek and edited a volume entitled Discriminant Analysis and Applications, published by Academic Press in 1973. In 1989, his book Exercises in Probability was published by Springer. During June 3–6, 1999, an International Conference was organized at the University of Athens in honor of Cacoullos and many of the eminent statisticians mentioned above were physically present in Athens on that occasion. A volume entitled Probability and Statistical Models with Applications that was prepared based on that conference, edited by Professors Charalambides, Koutras, and Balakrishnan, was published by Chapman and Hall/CRC Press in 2001.

We all had the distinct pleasure and a great memory of interacting with him in different capacity, and enjoyed our interaction and collaboration with him immensely for his incisive comments and also his great sense of humor. So, his passing came as a great personal as well as professional loss to all of us, and also the community at large.

To show our great appreciation for all the fine academic and professional contributions Theo had made through his illustrious career and life, we undertook the preparation of this special issue in his honor. We sought articles from various international researchers who had either been in correspondence with him and/or been active in areas of research Theo had contributed immensely to. We were indeed mighty pleased that most of whom we invited accepted our kind invitation to participate in this special issue. In this regard, we express our sincere thanks to all of the authors for presenting their articles for inclusion in this special issue and to the reviewers as well who gave their valuable time and effort to complete the review work in a timely fashion! Our special thanks are also due to Professor Sat Gupta, Editor-in-Chief, for accepting our proposal for this Special Issue to be published in Journal of Statistical Theory and Practice.

With all these fine contributions, we have thus been able to produce such a fine Special Issue in fond memory of our friend, Professor Theophilos Cacoullos.