Currently, there is no doubt that the first choice of treatment for alarming infantile haemangiomas (IHs) is oral beta-blockers. However, research in this field remains active, as the pathogenesis of IH is still not completely elucidated. Furthermore, there are different approaches to the management of IHs with beta-blockers. In Part 1 of this review we will discuss the state-of-the-art evidence for IH with regard to (i) the definition, epidemiology, course, risk factors and sequelae, and (ii) the pathogenesis, focusing on genetic studies. This review will update the reader on the latest developments in the pathogenesis of IH. Furthermore, we hope this review will give more insight into risk factors and sequelae of IH, thereby contributing to better decisions in the clinical management of patients with IH. The therapy and evaluation of IHs will be discussed in Part 2 of this review.