The WHO Reporting System for Lung Cytopathology is Volume 1 of this new series of reporting systems for cytopathology, which is a joint project of the International Academy of Cytology (IAC) and the International Agency for Research on Cancer (IARC), a specialized agency of the World Health Organization (WHO). The series includes a unique synthesis of the published evidence and the practice of cytopathology, and it is linked to the WHO Classification of Tumours series, now in its 5th edition. Cytopathologists look at tumours slightly differently than other specialists do, and there is a need for specialized reporting systems based on the key diagnostic cytopathological features of tumours, presented in standardized reports, within a hierarchical system of diagnostic categories. These categories must also be linked to diagnostic management recommendations to improve communication with clinicians and support patient care. And it is essential that these reporting systems be truly international, to serve the needs of patients worldwide in many differently medically resourced settings.