Investigational group  Inclusion Criteria:  participant in the FUTURE study diagnosed with type 1 diabetes ≥3 months before start of the FUTURE study use of insulin pens for the entirety of the FUTURE study used Insulin Detemir (Levemir®), Insulin Glargine 100 U/mL (Lantus®) or an equivalent as basal insulin before switch to Gla-300 (Toujeo®) switched to Gla-300 (Toujeo®) during the FUTURE study for a sufficient amount of time (at least two consecutive diabetes consultations = 8 months)  Exclusion Criteria:  use of Fast-acting insulin aspart (Fiasp®) before or after switch to Gla-300 not using bolus insulin (only basal insulin) for women: pregnant or planning pregnancy during the FUTURE study Control group  Inclusion Criteria:  participant in the FUTURE study for at least 18 months diagnosed with type 1 diabetes ≥3 months before start of the FUTURE study use of insulin pens for the entirety of the FUTURE study use Insulin Detemir (Levemir®), Insulin Glargine 100 U/mL (Lantus®) or an equivalent as basal insulin during the entirety of the FUTURE study  Exclusion Criteria:  use of Fast-acting insulin aspart (Fiasp®) not using bolus insulin (only basal insulin) for women: pregnant or planning pregnancy during the FUTURE study switch to Gla-300 (Toujeo®) or Insulin Degludec (Tresiba®) before 18 months in FUTURE study (if switched after 18 months, only data up to switch to Gla-300 [Toujeo®] or Insulin Degludec [Tresiba®] will be used for this control participant)