Eosinophils are granulocytes and belong to the innate arm of immunity. Eosinophils can be in different basal or activation states and depending on which type of activation is applied, they exert different effector functions. These multi-functional cells have been shown to play crucial roles in different types of cancer. In breast cancer patients, blood and tumor associated eosinophils were shown to have different effects on tumor progression, ranging from beneficial through neutral to detrimental. Several pre-clinical models showed both pro- and anti-tumorigenic roles of eosinophils which act either directly or indirectly through modifications of the tumor immune environment. The goal of my thesis was no analyze the effects of breast cancer associated eosinophilia on cancer progression using orthotopic tumor injections as this type of breast cancer model has been rarely used to analyze how eosinophils affect breast cancer progression. I first established a reliable way to identify eosinophils in tumor tissues and distinguish them from other immune cells, namely neutrophils. I did so by testing different gating strategies using flow cytometry and confirming my results with histological analyses. I tried several methods to attract eosinophils to primary tumors in mice and activate them in situ so as to analyze their effects on breast cancer progression. Strong eosinophilia in both the blood and the tumor was accomplished by treating the mice with an engineered version of IL5. IL5 is the main differentiation and survival factor of eosinophils and was coupled to the targeting moiety of an antibody against periostin, a protein expressed in the extracellular matrix in breast tumors and the resulting immunocytokine was termed AB5-IL5. This coupling allowed for increased circulation time as well as specific targeting to the tumor, resulting in stronger blood and tumor eosinophilia as compared to treatment with IL5 alone. I observed consistent increases in tumor associated eosinophilia in four different breast cancer models and my results indicated that eosinophilia in breast cancer has no effect on cancer progression when tumors were injected orthotopically. I also analyzed the tumor microenvironment in eosinophilic tumors and saw that eosinophils are able to affect the infiltration of tumor associated neutrophils, having an inhibitory effect on these cells. Furthermore, I discovered a yet unknown type of SiglecF+ neutrophil in 4T1 tumors which strongly resembled eosinophils and are most likely not the SiglecF+ neutrophils described by others in mouse models of lung adenocarcinoma. Overall, I was able to establish both a reliable gating strategy to identify eosinophils in different breast cancer models as well as consistently induce eosinophilia in both the circulation and breast tumors. I found that identification of eosinophils, especially in patient data, is more difficult than initially assumed and that conclusions drawn in studies using gene expression to identify eosinophils must be interpreted with care. I also discovered that pre-clinical studies about eosinophils in primary breast cancer are virtually non-existent. My results indicate that eosinophilia has no effect on primary tumor growth nor metastatic seeding in breast cancer but that these cells interact with other immune cells in the tumor microenvironment, mainly with neutrophils.