The IEEE Sensors Journal was born on June 1, 2001, when its first ever issue was published. The journal has grown exponentially over the past 20 years to become one of the world's largest journals in sensor engineering and technology as well as one of the largest among more than 200 IEEE publications. In these 20 years, the field of sensors has dramatically changed of course: Completely new areas have emerged, e.g., body sensor networks, the Internet-of-Things, memristive sensors, or the smart sensory dusts, while some others have enormously grown, e.g., mechanical sensors, fiber-optics sensors, or radiation sensors. In all these cases, the IEEE Sensors Journal has recorded and presented the progress and achievements of the field. As any engineering journal, IEEE Sensors Journal has also served the community by reporting extremely useful technical papers, in order to provide experiential knowledge. As well, the journal has published scientific manuscripts proposing new detection principles and novel sensing devices. After 20 years of activity, it is now the right time to look back critically at what the field has produced in terms of great achievements and, at the same time, to look ahead at the challenges for the next 20 years to come. That is the main aim of this Special Issue published to celebrate the 20th anniversary of the IEEE Sensors Journal. With this introductory article, we would like to give an overview of the past 20 years by commenting on their highly successful developments.