The extant literature shows that companies are willing to transition into sustainability and the circular economy (CE) yet lack the knowledge and skills to transform their actions to meet sustainability challenges in a meaningful way. Indeed, for companies to act in an environmentally responsible way, they must know enough about environmental issues, yet literature on this knowledge standpoint is missing. This paper assesses the knowledge and practices among manufacturing companies working towards sustainability and CE transitions. The study employs a survey design and in-depth interviews with 20 firms from the manufacturing industry in Norway. This study contributes to the theoretical understanding of sustainability and CE transitions by providing insight into the knowledge and practices of manufacturing firms, which may at the same time facilitate decision-makers in planning for the development of a CE and promote the sustainable development of the manufacturing industry in Norway.