Understanding the pharmacology of insulin and how it relates to the pathophysiology of diabetes can lead to better clinical outcomes. No insulin formulation should be considered “best” by default. Insulin suspensions (NPH, NPH/regular mixes, lente, and PZI) as well as insulin glargine U100 and detemir are intermediate-acting formulations that are administered twice daily. For a formulation to be an effective and safe basal insulin, its action should be roughly the same every hour of the day. Currently, only insulin glargine U300 and insulin degludec meet this standard in dogs, whereas in cats, insulin glargine U300 is the closest option.