This paper provides an introduction to Welfare Impact Assessment (WIA). We proposed the WIA as a new kind of impact assessment which can identify the social vulnerable consequences of a proposed policy and program, giving particular attention to the mitigation of adverse or unintended aspects. WIA can be defined as the consequences to the social vulnerable people of any proposed action that change the way they live, work, relate to one another, and function as individuals and members of society. To get guidelines about how to institutionalize WIA, we choose some related impact assessments such as Social Impact Assessment, Environmental Impact Assessment, Gender Impact Assessment, and Equity Impact Assessment and compare them. We also explain differences between WIA and other IAs. After that, several factors are presented which should be prepared for institutionalizing WIA. We hope that many scholars and IA professions give helpful comment and advice.