Monocytes/macrophages play a significant role in the host’s response to tumours. This includes: cytotoxic/cytostatic activity, presentation of tumour-associated antigens and induction of specific anticancer response of lymphocytes. Circulating blood monocytes respond to a gradient of chemoattractants produced by the tumour, migrate out from the blood to the tumour bed and form a large part of the cellular infiltrate as tumour infiltrating macrophages (TIM). Monocytes and macrophages produce a large array of factors (cytokines, reactive oxygen and nitrogen intermediates, growth factors, prostacyclins, ect.) with opposing biological activities. Consequently, TIM exhibit both tumour growth promoting and inhibitory activities. Furthermore, tumour-derived molecules also modulate TIM activity. In some circumstances monocytes/macrophages are involved in the metastatic process. This review summarizes the current state of knowledge in this area indicating that in fact macrophage-tumour interactions are quite complicated and a delicate balance exists between antitumour response and protumour effect of TIM and the suppression of TIM activity by the tumour. The clinical implications of these findings are also discussed.