This paper presents the findings of the Weighing up the Odds study which investigated the sports-betting motivations, attitudes and behaviours of young men aged 18-35 who watch or play sports. One quarter (23%) of bettors reported being under 18 when they first placed a bet on sports. Betting agency promotions were found to drive gambling uptake. Alcohol featured prominently in betting behaviour, with 64% of young men saying they had bet on sports while drinking. Of all young men who bet on sport, 70% were found to be at risk of, or already experiencing, gambling harm.