The continuing growth of internet services such as streaming videos, browsing websites or generally exchanging data has drawn the attention of academic researchers, industry, and the general public towards their environmental impact. Past assessments of this impact come to differing results due to the complexity of information and communication technology systems, including networks, data centres and user devices. Assuming a life-cycle perspective, this thesis reduces some of this uncertainty and thus works towards more robust assessments and ultimately decision-making. The first part of this thesis consists of modelling the energy consumption of routers and fibre-optical equipment that comprise the networks. As a result, new estimates of the energy intensity of networks are made, that can be used to derive the energy consumption of data transfer through the network. In t he second part, the energy consumption by data centres and use devices is included , which combined give a comprehensive assessment of the system end-to-end. One chapter is dedicated to the detailed analysis of the varying environmental footprint between different user devices and types of media. A separate chapter then develops and show-cases a more integrated assessment for an complete digital service over one year and demonstrates several new approaches to reducing uncertainty around use r device and access network energy consumption. The methods and models presented in this thesis are applicable to a wide range of services and contribute to more robust estimates of the energy consumption. The aim is to enable sustainability practitioners to carry out environmental assessments of digital services.