Balancing the innate immune system in tumor development

•The outcome of the innate immune system is determined by the tumor context.•The balance of innate immune cells dictates the fate of cancer development.•Innate lymphoid cells are important in regulating tumorigenesis.•There is an emerging role for neutrophil subsets in tumor progression.

Cells of the innate immune system have a dual role in cancer development in both tumor initiation and progression. Innate immune cells can, on the one hand, aid malignant transformation and tumor outgrowth and, on the other hand, prevent tumor progression. The innate immune system has the ability to tune the inflammatory response and is a key player in cancer-related inflammation, which can precede the development of malignancy or be induced by oncogenic changes promoting a protumor inflammatory milieu. In this review, we discuss the emerging cellular and molecular mechanisms of the innate immune system and inflammation in tumor initiation and progression, and point to the outstanding questions that remain.