Even in the era of genomic selection, relevant phenotypes are still needed and therefore phenotyping through the precise monitoring of the status of the cows, and their health, behaviour, and well-being as well as their environmental impact and the quality of their products is essential. The arrival of mid-infrared (MIR) based phenotypes have made a significant impact, as they can be obtained cheaply and quickly, are usable on a large scale, and are robust and reliable. This chapter covers the state of the art, the opportunities, and also the issues that need to be addressed in order to allow the even more successful use of MIR for genetic selection of dairy cattle. The authors cover different topics from the development of MIR-based prediction equations, their specificities in modelling, and their use in animal breeding. The latest developments and opportunities related to this novel technology for genetic and genomic selection in dairy cattle are also discussed.