Type 1 diabetes is characterised by the autoimmune destruction of pancreatic β-cells, leading to an absolute or near-absolute insulin deficiency. Although traditionally associated with childhood onset, it can manifest at any age, and it is increasingly recognised that there is significant heterogeneity in its clinical presentation. This review examines the intricate interplay between genetic susceptibility, environmental factors, and autoimmune mechanisms that contribute to the pathogenesis of type 1 diabetes. The role of clinical phenotype, along with diagnostic and clinical measurements of autoantibodies and C-peptide, in the classification of type 1 diabetes is discussed, alongside the challenges in diagnosing and classifying the disease. Emerging insights from studies into the heterogeneity of type 1 diabetes phenotypes and mechanistic endotypes underscore the need for refined diagnostic criteria, particularly in identifying autoimmunity in individuals initially diagnosed with type 2 diabetes. The impact of obesity and insulin resistance on disease progression and clinical management is also examined. Overall, this review aims to provide a comprehensive understanding of the evolving landscape of type 1 diabetes, highlighting critical areas for future research and potential therapeutic approaches tailored to individual patient profiles. PLAIN LANGUAGE SUMMARY: Type 1 diabetes is an autoimmune disease where the body does not produce insulin, leading to high blood sugar levels. Traditionally thought to start in children and young adults, type 1 diabetes can occur at any age. However, many factors contribute to when someone develops type 1 diabetes and how rapidly the disease progresses, including a person's combination of genetic factors, including specific genes that can either be protective or high-risk for the development of type 1 diabetes. Although it is widely assumed that a triggering event or events initiate the autoimmune process, the trigger or triggers remain unknown. However, it is this autoimmune process that causes progressive destruction of insulin-producing Β-cells in the pancreas that eventually leads to high blood sugar and the diagnosis of diabetes. Although we have several tools to diagnose and classify diabetes, including measuring autoimmune markers (antibodies) in the blood, there is significant variation in how individuals with type 1 diabetes can present, which can make recognizing and appropriately treating type 1 diabetes more challenging. Finding better ways to characterize the unique characteristics of each subgroup of individuals may provide new insights into how we can best tailor treatment to each of these patient groups.