After two decades of research effort, fine-pitched 3D integrated circuits are finally appearing in a growing range of industry products that leverage the benefits of high-bandwidth, high-density circuit integration. This article reflects on the historical development of these 3D technologies, their unique benefits over alternate 3D packaging methods, and the recent industry and research trends in 3D memories and 2.5D integration. Although die stacking is now happening, many potential benefits offered by these technologies remain to be explored, providing an opportunity for researchers and developers to solve these remaining challenges in architecture, methodology, and business.