Cellular senescence is a permanent growth arrest in cells with damage or stress 
that could lead to transformation. Some tumor cells also undergo senescence in 
response to chemotherapy. Senescent cells secrete cytokines and other factors of 
the senescence-associated secretory phenotype (SASP) that contribute to tumor 
suppression by enforcing arrest and recruiting immune cells that remove these 
damaged or oncogene-expressing cells from organisms. However, some cells can 
develop a SASP comprising factors that are immunosuppressive and protumorigenic 
by paracrine mechanisms. Likewise, the SASP in treated cancers can either 
contribute to durable responses or drive relapse. Here, we discuss the studies 
that have demonstrated a complex and often conflicting role for the SASP in 
tumorigenesis and treatment response.