Cancer spreads locally through direct infiltration into soft tissues or at distance by invading vascular structures, then migrating through the lymphatic or blood flow. Although cancer cells carried in the blood can end in virtually any corner of the body, lymphatic migration is usually stepwise, through successive nodal stops, which can temporarily delay further progression. In radiotherapy, irradiation of lymphatic paths relevant to the localisation of the primary has been common practice for decades. Similarly, excision of cancer is often completed by lymphatic dissection. Both in radiotherapy and in surgery, advanced knowledge of the lymphatic pathways relevant to any tumor location is an important information for treatment preparation and execution. This first part describes the major collecting trunks of the lymphatic system and then the lymphatics of the head and neck providing anatomical bases for the radiological delineation of lymph node areas in the cervical region, it adds to the existing nomenclature of six nodal levels (I-VI), three new areas listed as parotid, buccal and external jugular levels.