FiberJ is a Java program for processing x-ray fiber diffraction patterns. It transforms diffraction data into appropriate image representations for display, determines diffraction parameters, corrects systematic effects, maps diffraction data into reciprocal space, and provides various tools for structural analysis. In particular, FiberJ offers highly customizable colormaps great for visualizing patterns with large dynamic ranges of intensities. Its data plotting, image filtering and manipulation routines are also useful for general scientific image processing.