The tumor interstitial fluid (TIF) is a fluid phase present in the extracellular space of all tumors whose importance in oncology is seldom recognized. In order to stimulate other researchers to give it the due importance, a review of the available data (including our own) is provided. An hypothesis is presented for the genesis, fate and role of the TIF in the processes of invasion, growth and metastatization. Open questions regarding the TIF's role in tumor response to therapy are raised.