Slightly over 40 years ago, two pioneering papers appeared: one authored by Allan Sandage (1976, AJ, 81, 954), and the other one by Michael Disney (1976, Nature, 263, 573). The first reported an analysis of the so-called cirri, diffuse emission at very low surface brightness (LSB) levels which is produced by the scattering of stellar light by interstellar dust grains. This emission was shown to extend over the entire sky, and was detected in wide-field photographic plates using fast-optics telescopes. The second paper analysed the consequences, for extragalactic physics, of the biases introduced by surface brightness cuts. Indeed, the deep images obtained at the time (very modest for today's standards) were limited by the sky brightness, and hence introduced a bias on the apparent sizes and luminosities of the galaxies present in the images. This bias led, among other points, to the prediction of the existence of low surface brightness galaxies which could not be detected by the technology of the time.

In the 1970s, the low surface brightness sky was essentially a terra incognita, and thanks to the instrumentation of the time (Schmidt cameras) surveys were carried out, but the priority given to slow-optics telescopes aimed at resolved and point-like sources made the field of LSB progress far more slowly. Only recently, with the discovery of ultra-diffuse galaxies (see below) with fast-optics cameras (first by amateur astronomers and then by professionals), the field has re-emerged, with the realisation that the LSB sky is indeed the last niche that remains to be explored in optical/UV astronomy. The proposed symposium aims at celebrating the 43rd anniversary of these papers by analysing the impact that the low surface brightness sky has on a wide variety of topics ranging from the Solar System to the cosmological background radiation. Clearly there is a tremendously huge range of questions that will be addressed during the Symposium to elucidate these issues and envision the future of this exciting field.

The topics addressed in this Symposium go well beyond previous (somewhat) related symposia such as IAU S321 "Formation and evolution of galaxy outskirts" (2016), and IAU S317 "The General Assembly of Galaxy Halos: Structure, Origin and Evolution" (2015), and not have been addressed before comprehensively. In fact, while the existence of these low surface brightness features was found back in the 1970s, with the first detections on photographic plates, it is extraordinarily surprising that there has been only one single IAU meeting (Colloquium 171,"The low surface brightness universe", Cardiff), back in 1998, devoted to the subject.

Twenty one years later, the time is ripe for this Symposium, reviewing the properties of what remains, by and large, the last remaining niche to be explored in observational parameter space.