The past 20 years can be regarded as the adolescence of geographic information science (GIS), as it grew from a burgeoning area of study into a mature and thriving field. During those two decades, the International Journal of Geographic Information Science (formerly Systems) (IJGIS) was one of the most prominent academic guiding forces in GIScience, and looks to remain so for the foreseeable future. To celebrate this important milestone, Peter Fisher-the second editor of IJGIS-has compiled 19 of the most significant and influential articles ever published in the journal. "Classics from IJGIS: Twenty Years of the International Journal of Geographical Information Science and Systems" contains articles that shaped the way we approach the science of GIS, provided seeds from which grew key developments that have moulded the field, opened new avenues of discovery across the landscape of modern GIScience, and some that may mark how it will change in the future. Accompanying nearly every article is a commentary written by at least one of the original authors. In these commentaries, the authors reflect on various aspects of the original work, including what drove their original research, why the concepts were widely adopted (or in some cases, why not), and what the future may hold for new research embarking on these and similar areas. The 20th article in this collection is an original contribution that examines the social and collaborative networks operating within and shaping the body of research in the field.