The incidence of radiation pneumonitis (RP) has decreased significantly compared to historical series, mainly due to improved radiotherapy techniques and patient selection. Nevertheless, some patients still develop RP. This guideline provides user-friendly flowcharts to address common clinical practice questions regarding RP. We summarize the current state of the art regarding the mechanisms, risk factors, diagnosis and treatment of RP. Dosimetric constraints to minimize the incidence of RP, as well as risk factors for developing RP, such as idiopathic pulmonary fibrosis (IPF) were identified. The combination of radiotherapy and medication as a risk factor for the development of RP was reviewed. RP remains a diagnosis of exclusion, but an algorithm for reaching the diagnosis has been proposed. Finally, practical approaches to the treatment of RP are outlined.