In this article we present the role and discuss the significance of monocytes in both innate and adaptive immunity. Monocytes are the largest white blood cells of convoluted, bilobed nuclei. Growth factors and cytokines determine the monocyte subtype. The monocyte functional and phenotype heterogeneity have been recognized and in the recent years there have been identified their major populations in humans; classical (CD14+CD16−), non-classical (CD14dim CD16+), and intermediate (CD14+CD16+). Each of these subpopulations is distinguished by the expression of distinct surface markers and by their functions in homeostasis and disease. Monocytes develop in bone marrow and travel to tissues and organs in where they become macrophages or dendritic cells. Macrophages are effector cells of the innate immune system that phagocytose bacteria and secrete both pro-inflammatory and antimicrobial mediators. In addition, macrophages play an important role in eliminating diseased and damaged cells through their programmed cell death. Generally, macrophages ingest and degrade dead cells, debris, tumor cells, and foreign materials. Dendritic cells represent a heterogeneous family of immune cells that link innate and adaptive immunity. Their main function is to process antigen material and present it on the cell surface to the T cells of the immune system. Monocytes differentiated into dendritic cells play an important role in innate and adaptive immunity, due to their microbicidal potential, capacity to stimulate CD4+ and CD8+ T-cell responses and ability to regulate immunoglobulins production by B cells.