Over the past decades, obesity became a tremendous socioecological problem, reaching epidemic dimensions and the number of accompanying comorbidities including type 2 diabetes and cardiovascular diseases can be extended by different types of cancer. The small peptide apelin is involved in key physiological processes such as angiogenesis, fluid homeostasis, and cardiovascular function. As a relatively new adipokine, apelin also exhibits metabolic functions in regulating insulin sensitivity and glucose metabolism. In rodents and humans, apelin serum levels seem to correlate with the nutritional status and even more interesting, apelin expression levels are increased in many different cancers due to its proangiogenic capacities, supporting a role of apelin as diagnostic biomarker in cancer and its potential use in anticancer therapies by blocking tumor neovascularization. This chapter highlights the role of apelin in both physiological and pathological conditions like tumorigenesis, especially in brain tumor development and reviews apelin signaling in metabolic disorders like obesity-related malignancies, to identify possible associations between the adipokine apelin and tumor progression.