Fast X-ray transients are brief (~seconds) and intense bursts of high-energy radiation that punctuate the otherwise steady X-ray sky, appearing and fading on timescales from seconds to minutes. Although dezens of such events have now been detected, their physical origins remain surprisingly diverse and, in many cases, poorly understood. Some fast X-ray transients arise within our Galaxy, often associated with a nearby stellar progenitor, while others appear to be extragalactic and do not fit neatly into the known and established classes of high-energy transients.
In this talk, I will review recent observational progress in the study of fast X-ray transients. I will highlight what we have learned from well-studied sources, discuss emerging evidence for new populations of extragalactic fast X-ray transients, and examine how these events compare to other transient phenomena across the electromagnetic spectrum.