Can Big Data improve understanding of well-being and can they harm well-being? The chapter opens by asking what even is ‘Big Data’, and is ‘it’ actually new when large datasets have been valuable in understanding population-level health, wealth and well-being for 6000 years. It reviews the failed promises of Big Data to predict and prevent pandemics, including COVID-19, comparing new data infrastructures with old ones. It presents examples and case studies of social media data and data mining on large scales, and for smaller organisations to understand how we feel. We find there are more limits to Big Data and new data technologies to understand well-being than are made explicit, and question the ethics of Big Data insights and their monetary value in the context of well-being.