Some flavors of Linux make ImageJ 1.x available as a package. Do not install ImageJ this way—you will not receive ImageJ2, and you will have problems installing additional plugins. For the time being, it is recommended to download the Linux 64-bit distribution of Fiji.
You can make ImageJ 1.x scale better on HiDPI displays by installing a Java 9 prerelease. See this forum post. Unfortunately, ImageJ2 and Fiji do not yet launch using Java 9, due to limitations of the ImageJ Launcher and ImageJ Legacy components.
See the Troubleshooting page.
Frequently Asked Questions
See the Frequently Asked Questions page.