ImageJ is public domain software for processing and analyzing scientific images, with many derivatives and variants, including ImageJ2, Fiji, and others.

The original ImageJ has been under continuous development since 1997. It has always been, and continues to be, a project of Wayne Rasband, with the help of many contributors. It was initially developed in 1997 as a cross-platform version of NIH Image. ImageJ grew organically over time as Wayne Rasband continued to add features according to user requests. Now there are many hundreds, probably thousands, of plugins written by members of a diverse community.

The other main flavor of ImageJ, a redesign started in 2010, is known as ImageJ2, to differentiate it from the original ImageJ. The Fiji distribution of ImageJ bundles both ImageJ 1.x and ImageJ2, including ImageJ2’s backwards compatibility layer to transparently convert between ImageJ 1.x and ImageJ2 data structures as needed.