In the Netherlands, young adults’ drinking practices have become an issue of public concern since their drinking levels are high. Heavy drinking can place young adults at an increased risk for developing short and long-term health-related problems. Current national alcohol prevention programmes focus mainly on adolescents and their parents and paying less systematic attention to young adults. The present study describes the theory and evidence-based development of a web-based brief alcohol intervention entitled What Do You Drink (WDYD). We applied the Intervention Mapping (IM) protocol to combine theory and evidence in the development and implementation of WDYD. The WDYD intervention aims to detect and reduce heavy drinking of young adults who are willing to decrease their alcohol consumption, preferably below the Dutch guidelines of low-risk drinking. According to the IM protocol, the development of WDYD resulted in a structured intervention. Reducing heavy drinking to low-risk drinking was proposed as the behavioural outcome. Motivational interviewing principles and parts of the I-Change Model were used as methods in the development of WDYD, whereas computer tailoring was selected as main strategy. An effect and a process evaluation of the intervention will be conducted. IM was found to be a practical instrument for developing the WDYD intervention tailored to a specific target population in the area of alcohol prevention.