Version 4 of the 3D Slicer application, created and supported by NAC for nearly two decades, has surpassed the 260,000 download milestone since it was released in 2011. The most recent version 4.6.2 has been downloaded at a rate of over 290 downloads per day in the first two and a half months of 2017, with a clear trend toward steadily a steadily increasing download rate. These download statistics are evidence of the worldwide impact of the free and open source 3D Slicer software and the valuable part it plays in enabling science and clinical research across a range of applications. In addition to core functionality in image visualization, analysis, segmentation, and registration, 3D Slicer is fully extensible and programmable. As a software platform, 3D Slicer includes an 'app store' with hundreds of add-on software modules developed by investigators at dozens of academic and commercial sites. Specialized research communities have formed in over a dozen domains to create and support software customized to the needs of their work. Very active and productive multi-institution research teams such as the SlicerRT community for radiation therapy research, the SliceCMF team developing dental and craniomaxillofacial surgery software, SlicerDMRI for diffusion imaging, and the SlicerPathology team developing analysis tools to study digitized histology images are prime examples of the diverse research that can build around a common software base. The NAC Slicer Core continues to help steer the world-wide community of 3D Slicer users and developers while working to improve the core architecture to address the next generation of imaging software research challenges.