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Simple Summary: The body's defense system, which comprises immune cells, is an important factor contributing to tumor growth and treatment response. A "hot" tumor, infiltrated by various beneficial immune cells, usually corresponds with a better prognosis. A so-called "cold" tumor that lacks these beneficial immune cells or that contains harmful cells that block an immune response often results in a harder fight against cancer. Turning a "cold" tumor into a "hot" tumor may help cancer treatments, like immunotherapy, work better. In this study, we looked at the immune cell environment in lung, colorectal, and skin cancer. Tumors were grown in mice, and the different types of immune cells were studied using a fluorescent technique. We found differences in the immune cell infiltration of the diverse tumor types, with some tumors being more "hot" and others showing a tendency to be "cold". These findings can help us understand how various tumor types interact with the immune system. The tumor microenvironment (TME) is pivotal in cancer progression and the response to immunotherapy. A "hot" tumor typically contains immune cells that promote anti-tumor immunity, predicting positive prognosis. "Cold" tumors lack immune cells, suggesting a poor outlook across various cancers. Recent research has focused on converting "cold" tumors into "hot" tumors to enhance the success of immunotherapy. A prerequisite for the studies of the TME is an accurate knowledge of the cell populations of the TME. This study aimed to describe the immune TME of lung and colorectal cancer and melanoma, focusing on lymphoid and myeloid cell populations. We induced heterotopic immunocompetent tumors in C57BL/6 mice, using KP and LLC (Lewis lung carcinoma) cells for lung cancer, MC38 cells for colorectal cancer, and B16-F10 cells for melanoma. Immune cell infiltration was analyzed using multicolor flow cytometry in single-cell suspensions after tumor excision. KP cell tumors showed an abundance of neutrophils and eosinophils; however, they contained much less adaptive immune cells, while LLC cell tumors predominated in monocytes, neutrophils, and monocyte-derived dendritic cells. Monocytes and neutrophils, along with a significant T cell infiltration, were prevalent in MC38 tumors. Lastly, B16-F10 tumors were enriched in macrophages, while showing only moderate T cell presence. In conclusion, our data provide a detailed overview of the immune TME of various heterotopic tumors, highlighting the variabilities in the immune cell profiles of different tumor entities. Our data may be a helpful basis when investigating new immunotherapies, and thus, this report serves as a helpful tool for preclinical immunotherapy research design. [ABSTRACT FROM AUTHOR]