In this brief review, we report on the state of knowledge and its limitations regarding the environmental footprint of the digital sector. While the final energy consumption and the related carbon footprint of information and communication technologies (ICT) are well studied, other environmental factors and some phases of the life cycle are still poorly understood. New connected equipment and services currently being deployed make such assessments even more complex. In addition, more research is needed on the indirect effects of ICT (i.e. substitution, optimization and various rebound effects). Indeed, recent reports tend to ignore or minimize the negative effects of digitization. Because indirect effects depend on external factors such as regulations, prices, socio-cultural contexts, etc., extrapolations are extremely uncertain. Methodological elements have been proposed to perform such evaluations, but there is still a long way to go.