The rules that govern tumour treatment depend largely on clinical stage (tumour volume, localization and/or metastasis presence). These rules are applied assuming that tumor growth is relatively static without considering the time factor, the number of clonogenic cells in the tumour or the volume reduction following initial cytotoxic therapy. Time and neoplastic growth (with a subsequent change in volume) are generally not considered in 90% of clinical trials, where chemotherapy is administered on the first and eighth day and radiotherapy is carried out five days a week with different schedules. In the clinical situation, however, a tumour has more complex growth times that should be appropriately assessed to improve the treatment results (1). The aim of this paper is to stress the influence of the time factor to optimize the schedule of the cytotoxic therapies, based on different mathematical models developed to describe the tumour growth. To better understand the role of the neoplastic growth at its different clinical stages and the subsequent response to cytotoxic therapies, several elements concerning such growth should be thoroughly analyzed.