This thesis looks at HD opera events, with the purpose of revealing how they reshape our understanding and usage of the concept of liveness. The popularity of The Met: Live in HD series in the last decade inspired other art companies (theatre, ballet, and even museums) to follow the same model and broadcast live in movie theatres. Together they seem to have shifted both our understanding of, and attitude towards, liveness. The discussion about liveness in the last century has revolved around the distinction between live and recorded, and around the co-dependent opposition between original and copy. Recently, liveness and the discourse about it have been influenced by a now blurry distinction between live and recorded, and by the emergence of online media. The emergence of HD events, which reposition liveness in a rediscovered environment – the movie theatre –, brings liveness to a new theoretical impasse, which I am tackling in my thesis. My investigation follows liveness through the lens of the long debated relationship between classical music and technology. I approach this relationship by having a close look at the production and reception of live broadcast events, starting with the radio in the 1920s, and focusing on HD opera as a new medium. Furthermore, I identify trends in how liveness is constructed, evaluated and perceived, taking into account the fact that over the decades, the understanding of liveness evolved with media usage, and also within its cultural and social context. My research shows that liveness as a concept does not stand and evolve only in relationship with the production and reception of live broadcast events, and as constructed by broadcasting media, but that it is rather continuously shaped and redefined by its rapport with what I call liveness practice, a set of expectations and practices related to liveness that are common to all mediated events, and acknowledged by consumers as such. I claim that, by encompassing public venues, HD events expand our concept of liveness practice. Consequently, they render liveness in classical music as a fully accepted alternative to live performance.