The process of lymph node metastasis is not completely understood. Although we know some basic molecular mechanisms of metastasis, the exact procedure from the initiation of a primary tumour to overt lymph node metastasis remains obscure. A few morphological features of different primary tumours are known to correlate with the probability of lymph node metastasis, e.g., tumour histology (carcinoma vs. sarcoma), tumour size, T category, poor grade of differentiation and invasion of lymph vessels. Few attempts have been made to use markers of molecular differentiation such as nm23 as an additional indicator of lymph node metastasis. A drawback is the lack of an exact definition of lymph node metastasis or micrometastasis and how to include new findings such as the demonstration of cytokeratin-positive cells in "tumourfree" lymph nodes. Some of these aspects will be discussed within this review and proposals for classifications presented.