This book is a snapshot of the ether qua epistemic object in the early twentieth century. It shows that the ether was not necessarily regarded as the residue of old-fashioned science, but often as one of the objects of modernity, hand in hand with the electron, radioactivity or X-rays. Instrumental in this was the emergence of wireless technologies and radio broadcasting, which brought the ether into social audiences who would otherwise have never heard about it. Following the prestige of scientists like Oliver Lodge and Arthur Eddington as popularisers of science, the ether became common currency among the general educated public. Modernism in the arts was also fond of the ether in the early twentieth century: the values of modernism found in the complexities and contradictions of modern physics provided a fertile ground for the development of new artistic languages, in literature as much as in the pictorial and performing arts. The question of what was meant by ‘ether’ (or ‘aether’) in the early twentieth century at the scientific and cultural levels is also central to this book. The chapters in this book display a complex array of meanings that will help elucidate the uses of the ether before its purported abandonment. Rather than considering ether as simply a term that remained popular in several groups, this book shows the complexities of an epistemic object that saw, in the early twentieth century, the last episode in the long tradition of stretching its meaning and uses.