This multi-center randomized control trial (RCT) will compare efficacy and safety endpoints using the insulin-only configuration of the iLet Bionic Pancreas (BP) System versus Usual Care (UC) during a 13-week study period. Participants may be enrolled initially into a screening protocol and then transfer into the RCT protocol, or they may enter directly into the RCT protocol. The RCT will be followed by an Extension Phase in which the RCT Usual Care (UC) Group will use the insulin-only configuration of the iLet Bionic Pancreas (BP) System for 3 months. At the completion of use of the BP system in the RCT only, participants will enter a 2-4 day Transition Phase and be randomly assigned to either transition back to their usual mode of therapy (MDI or pump therapy) based on therapeutic guidance from the iLet BP System or transition back to their usual mode of therapy based on what their own insulin regimens were prior to enrolling in the RCT. There is an optional ancillary study to assess the safety of utilizing self-monitored blood glucose (SMBG) measurements instead of continuous glucose monitor (CGM) measurements as input into the iLet for ~48-60 hours. The Study is intended to mirror a real-world situation where CGM may not be available for an extended period of time (eg, user runs out of sensors and is awaiting new shipment).