The “International Principles for Social Impact Assessment” and the “Principles and Guidelines for Social Impact Assessment in the USA”, both developed under the auspices of the International Association for Impact Assessment and published in 2003, are compared. Major differences in the definition and approach to social impact assessment (SIA) are identified. The US Principles and Guidelines is shown to be positivist/technocratic while the International Principles is identified as being democratic, participatory and constructivist. Deficiencies in both documents are identified. The field of SIA is changing to go beyond the prevention of negative impacts, to include issues of building social capital, capacity building, good governance, community engagement and social inclusion.