Background: Over the past two decades, insulin glargine 100 U/mL (Gla-100) has emerged as the "standard of care" basal insulin for the management of type 1 diabetes mellitus (T1DM). Both formulations, insulin glargine 100 U/mL (Gla-100) and glargine 300 U/mL (Gla- 300) have been extensively studied against various comparator basal insulins across various clinical and real-world studies. In this comprehensive article, we reviewed the evidence on both insulin glargine formulations in T1DM across clinical trials and real-world studies.