BACKGROUND 1. New parameters of glycaemic control by use of continuous glucose monitoring (CGM; such as time in range, glycaemic variability) are of increasing interest in type 1 diabetes (T1DM) clinical practice. Poor glycaemic control negatively affects arterial stiffness, a valuable predictor for cardiovascular disease. However, reliable data using the state-of-the-art methodology (i.e. CGM) are missing. 2. Easy implementable strategies to improve glycaemic control and arterial stiffness are needed. Postprandial interval exercise might be effective but optimal regimens remain unknown. AIMS 1. Examine relation between CGM-derived glycaemic control and arterial stiffness in T1DM. 2. Determine the effects of postprandial high-intensity interval exercise (HIIE) on CGM-derived glycaemic control. 3. Investigate the effects of a long-term HIIE training program on arterial stiffness, glycaemic control and exercise capacity. METHODOLOGY WP1: Cross-sectional study (n=60). Assessment of glycaemic control by CGM for 7 days; arterial stiffness non-invasively by carotid-femoral pulse wave velocity (cfPWV) and pulse wave analysis; exercise capacity by maximal cardiopulmonary exercise test WP2: Acute cross-over RCT (n=32): Comparison of 3 postprandial HIIE bouts (performed ½, 1 or 2h after meal) to impact postprandial glycaemic control WP3: Chronic cross-over RCT (n=12): A 12-week supervised and home-based HIIE program to improve arterial stiffness, glycaemic control and exercise capacity