One of the major obstacles in clinical oncology is that tumors may relapse, resist therapy and sometimes even spread despite an initial successful treatment. We have pioneered the idea that host effects generated in response to therapy are substantially involved in tumor resistance and could even lead to metastasis spread. We termed this phenomenon as “Host Response to Anti-Cancer Therapy”. Our findings provide a larger view of the balance between anti-tumorigenic and pro-tumorigenic effects of cancer treatments, and therefore can pave the way for the development of novel treatment strategies.