Regular exercise improves fitness and reduces cardiovascular disease risk in people with type 1 diabetes (T1D). Clinical management recommends >150 min moderate intensity exercise per week. However, few patients achieve this, primarily due to a lack of time, fear of hypoglycaemia and inadequate knowledge around exercise management. High intensity interval training (HIT) is more time efficient than moderate intensity continuous training (MICT) and leads to similar improvements in aerobic capacity (VO2max) and metabolic health in non-diabetic controls. Here we investigated: 1) whether HIT is a safe training mode for people with T1D, limiting hypoglycaemic risk both during exercise and in the nocturnal period following exercise and 2) whether 6 weeks of HIT improves markers of cardio-metabolic health in comparison to MICT. The investigation was split into 2 parts. In Part 1, 14 participants with T1D (6M/8F; age 26±3 yr; BMI 27.6±1.3 kg/m2; duration of T1D 8.2±1.4 yr) on a basal-bolus insulin regimen completed a randomised counterbalanced crossover design whereby continuous glucose monitor systems (CGMS) were used to assess glycaemic control and risk of hypoglycaemia following HIT (6x1min cycling at 100% VO2max, interspersed with 1min rest) and MICT (30min continuous cycling at 65% VO2max) on separate days, in comparison to a control day of no exercise. Time in euglycaemia (4-11mM), hypoglycaemia (<4mM) and hyperglycaemia (>11mM), and number of hypoglycaemic episodes over each 24h period were measured. In Part 2, 14 previously sedentary people with T1D (10M/4F; age 29±3 yr; BMI 27.3±1.0 kg/m2; duration of T1D 11.3±1.8 yr) completed 6 wks of HIT (3x/wk of 6-10x 1min cycling at 100% VO2max, interspersed with 1min rest) or MICT (30-50min continuous cycling at 65% VO2max). Changes in VO2max, aortic pulse wave velocity (aPWV) and 24h glucose profiles were measured from pre to post training. In Part 1, there was no difference in % time spent in hypoglycaemia between the 3 conditions over 24h (CON=6±1; HIT=8±3; MICT=5±2; P>0.513) or overnight (24:00-06:00; CON=9±5; HIT=8±4; MICT=8±5; P>0.978). Similarly, there were no differences in the incidence of hypoglycaemia over the 24h period (Con = 1.8 ± 0.4; HIT=2.2±0.6; MICT=1.6±0.5; P=0.337) or overnight (Con=0.6±0.3; HIT=0.8±0.4; MICT=0.6±0.2; P=0.837) between the three conditions. Six weeks of HIT and MICT improved VO2max and aPWV. Using CGMS, no changes in 24h glycaemic control were observed following training in either group. In summary, an acute bout of HIT does not increase the post-exercise hypoglycaemia risk in people with T1D and 6 wks HIT training is effective for improving aPWV and VO2max. As the latter is the most powerful predictor of longevity in the general population, we propose HIT as a safe and time-efficient alternative to MICT to increase cardiovascular health in T1D patients.