2(5H)-Furanone derivatives are a large family of heterocycles that include synthetically useful compounds, several natural products and drugs with diverse biological activities. From the 1960, there has been a continuous interest in the development of efficient methods for preparation of these heterocycles and in their synthetic applications. The studies in these fields have been summarised and discussed in several excellent reviews, some of which have been recently published. The present article aims at completing the picture of these studies and highlights the most recent results from our and other and our laboratories which have been considered only shortly in previous reviews or have not been reviewed at all. In particular, it deals with the preparation and reactivity of 3,4- dichloro-5-hydroxy-2(5H)-furanone (also called mucochloric acid) and 3,4-dibromo-5-hydroxy-2(5H)-furanone (also called mucobromic acid) and the synthetic applications of these inexpensive, highly functionalised synthons. These applications illustrate that these mucohalic acids can provide a simple and convenient entry to a wide variety of interesting organic compounds and are mainly concerned with the selective and efficient preparation of variously substituted 2(5H)-furanone derivatives, including natural and unnatural compounds, which exhibit antiinflammatory activity or are characterised by potent cytotoxic activities against human cancer cell lines. Nevertheless, our attempt to tell the “whole story” of mucochloric and mucobromic acids from 1873 up to the present day is also concerned with a discussion on the use of these substances for preparation of stereodefined acyclic unsaturated dihalogenated derivatives and sulfur- or nitrogen-containing heterocycles, including members, which are precursors to agrochemically important substances.