This chapter considers the long history of cross-modal theories of sound and image from both a theoretical and practical perspective. Looking at key ideas related to colour, tone, vibration, space, time and data, the chapter describes key points of audio-visual cross-fertilisation. The discussion starts with colour-pitch theories, from Isaac Newton in the 18th century to Alexander Wallace Rimington in the 19th. It then looks at how these theories evolved alongside a more spectral understanding of both phenomena through the work of Wassily Kandinsky, Margaret Watts Hughes and Arnold Schoenberg in the 20th century. It looks at how this understanding evolved as audio-visual phenomena became concrete media through technology and the work of Pierre Schaeffer, Mary Ellen Bute and John Whitney. In the 21st century it looks at digital and post-digital aesthetics across Kim Cascone, Yasanao Tone and Ryoji Ikeda. The chapter also covers the origins and workings of a wide variety of audio-visual devices from the same periods, including the Helmholtz Sound Synthesizer from the late 19th century, Evgeny Sholpo’s Variophone from the early 20th century and the various audio-visual devices (e.g. sound and video synthesizers) that are common in the late 20th and early 21st centuries. The chapter ultimately argues that the tools of technology have themselves become key to an aesthetic understanding of audio-visual culture.