The immune system is composed of a diverse range of cell types, each with a distinct function. It can be broadly divided into the lymphoid (T, B, NK, etc.) and myeloid (monocyte, granulocyte, etc.) arms. Lymphopoiesis, the development and differentiation of lymphoid lineages, has been studied extensively for decades. For example, the influence of extracellular signals, signaling pathways and transcription factors has already been well documented. However, the importance of microRNAs has been highlighted by a surge of studies in recent years. In this review, we will discuss what is currently known about the role of microRNAs in lymphopoiesis, from the hematopoietic stem cell through to the differentiation of mature lymphocytes including thymic development, helper and regulatory T cells, fate determination of B cells and dendritic cells.