Icing of solid surfaces is an ever-present problem for many engineering applications. In particular ice accretion due to the impact and freezing of supercooled water drops is rich in various physical processes and of relevance for aviation, road traffic, shipping, wind turbines, and high-voltage power lines and insulators. It is initiated by the impact of water drops being in a thermodynamic meta-stable state, followed by nucleation of the impacting drops and ending up with solidification of the liquid, potentially influenced by the impact surface. All of these processes have been separately the focus of extensive experimental, theoretical and numerical studies for many decades. However, for the first time the present work attempts to consolidate the understanding of all involved mechanisms, and to examine them in the context of icing of surfaces. Following the aforementioned subdivision, the subprocesses of ice accretion are separately examined using experimental, theoretical and numerical approaches. Non-isothermal impact of both supercooled water drops and water drops initially at room temperature onto surfaces below the freezing point of water is studied experimentally and numerically, and the heat transfer during drop impact is theoretically modeled. Nucleation during drop impact is experimentally studied and theoretically modeled. The freezing process of supercooled water drops with wall contact is investigated, employing a novel experimental approach, and the influence of the wall is theoretically modeled. Finally, the mutual influence between fluid flow during drop impact and freezing of the impinging liquid is experimentally examined and theoretically modeled. Due to its comprehensive nature and the application of new experimental approaches, the present work constitutes a fundamental contribution to a better understanding of the processes taking place during ice accretion by supercooled water drops. It provides theoretical models which allow the prediction of heat transfer during non-isothermal drop impact, the quantitative analysis of experiments aimed at nucleation during drop impact, the prediction of the characteristic solidification velocity for the case of water freezing with wall contact, and the prediction of the residual ice layer thickness after impact of an individual supercooled water drop.