Environmental software systems (ESS) are software systems supporting activities of environmental protection, environmental management, environmental policy and environmental sciences. ESS often overlap with adjacent application fields like security, agriculture, health or climate change. The ISESS conference series is one of several overlapping events which are devoted to environmental modelling and software systems. ISESS is probably the one international event which has the strongest focus on the software angle.This paper gives a historic perspective of 20 years of ISESS conferences. Starting with a historic review of the development of the field named “environmental software systems”, it puts the ISESS conference into the greater perspective of similar activities. Using the original materials the paper identifies typical themes subsumed under ESS, and highlights typical topics for the time.The paper is the only existing complete collection of activities of the IFIP working group 5.11 “Computers and Environment”. Material of all events, ISESS conferences as well as co-organised workshops has been collected in one central place and will also to be kept up to date in the future.