Diabetes mellitus is a syndrome characterized by insulin deficiency that leads to hyperglycemia. There are four major types of diabetes mellitus, including type 1. The type 1 diabetes mellitus results from destruction of pancreatic β-cells, usually leading to absolute insulin deficiency. The inflammatory lesion within islets related to type 1 diabetes mellitus (called insulitis) is typically characterized by a decrease (or absence) of insulin-producing β-cells along with a pancreatic islet cell infiltrate, composed of macrophages and other cells that represent the immune response. Macrophages are found in all tissues and show great functional diversity. They have roles in development, homeostasis, tissue repair and immunity. It has been shown that macrophages play an important role in type 1 diabetes mellitus: uptake, processing and presentation of antigens to T-lymphocytes and regulation of T-lymphocyte-driven responses through cytokine release. In this article, we review recent insights into the role of macrophages with regard to their functions and importance as antigen-presenting cells in type 1 diabetes mellitus.