Hessdalen is a small valley in the central part of Norway. At the end of 1981 through 1984, residents of the Valley became concerned and alarmed about strange, unexplained lights that appeared at many locations throughout the Valley. Hundreds of lights were observed. At the peak of activity there were about 20 reports a week. Assistant professor Erling Strand, the founder of the Hessdalen project, conducted together with the author the first international congress of the Hessdalen phenomena in 1994. This congress attracted scientists from all over the world and boosted the scientific research in Hessdalen. Lights are still being observed in the Hessdalen Valley, but their frequency has decreased to about 20 observations a year.The following is the conclusions from over 25 years of research and after Italian SETI scientists got involved with much more highly sophisticated equipment.