Pancreatic ß and a cells play essential roles in maintaining glucose homeostasis. However, the mechanisms by which these distinct cell populations are generated, expand, and mature during pancreas development remain unclear. In this study, we addressed this critical question by performing a single-cell transcriptomic analysis of mouse ß and a cells sorted from fetal to adult stages. We discovered that ß and a cells use different regulatory strategies for their maturation and that cell proliferation peaks at different developmental times. However, the quiescent and proliferative cells in both the ß lineage and a lineage are synchronous in their maturation states. The heterogeneity of juvenile ß cells reflects distinct cell-cycling phases, origins, and maturation states, whereas adult ß cells are relatively homogeneous at the transcriptomic level. These analyses provide not only a high-resolution roadmap for islet lineage development but also insights into the mechanisms of cellular heterogeneity, cell number expansion, and maturation of both ß and a cells. Overall design: The overall goal of this study was to define the roadmaps for pancreatic ß- and a-cell development. Specifically, we performed single-cell RNA-seq at various developmental stages of E17.5, P0, P3, P9, P15, P18 and P60 of ß- and a- cells (except P3), as well as endocrine progenitor cells at P0, which were fluorescence-activated cell sorting (FACS) sorted from Insulin-RFP, Gcg-Cre; Rosa-RFP or Ngn3-GFP mouse strains, respectively. To develop a workflow to decipher the maturation process through bulk-cell transcriptomic analysis, we performed RNA-seq using 3-5 × 10^4 sorted cells at various developmental time points as we have done in the single-cell study. The background strains of our mouse samples are: Ngn3-GFP mice: mixed background of C57BL/6 and C3H Ins1-RFP mice: mixed background of C57BL/6 and C3H Gcg-Cre, Rosa-RFP mice: mixed background of C57BL/6, CBA/J and C3H.