Background: The significant health problems associated with people with ID are widely documented and acknowledged. Despite the recognised high level of unmet health needs, many of the frontline staff who work with this population receive little health promotion training. A review of the literature found no validated tools/instruments to assess staffs' knowledge and attitudes with regard to promoting the health of people with ID. Aim: The aim of this study was to develop and test a questionnaire which will assess the knowledge, attitudes and practices of care staff in promoting the health of people with ID. Method: This is a four phase multi-method study: Phase 1 involved 13, one-to-one telephone interviews with international experts from the field of I D and health promotion focusing on the enablers and barriers of health promotion for this population. During phase 2, seven focus groups were undertaken with frontline staff, family carers and adults with ID to explore their knowledge, attitudes and practices regarding health promotion. The findings from these two phases coupled with the insights from a literature review were utilised in the development and testing of a questionnaire in phase 3. This questionnaire was first piloted with experts and a sample of ID staff. During phase 4 the finalised questionnaire was then tested with a sample of (N=248) staff on two separate occasions to examine its validity and reliability. Initially an exploratory factor analysis was carried out on the Time 1 data. This factor structure was then tested using confirmatory factor analysis on the Time 2 data. Findings: Six core themes were identified from phases 1 and 2 (staff knowledge, attitudes, work environment, education/training needs, roles and responsibilities and barriers to health promotion). The questionnaire was piloted and tested and found to be broadly reliable and valid. The questionnaire confirmed that staff generally had limited knowledge about the health and health promotional needs of this population. Many staff held negative attitudes. Staffs' roles and responsibilities were not clear, and their employing organisations did not have a strong health promotion ethos. Results also highlighted a lack of health promotion policy. Discussion: This new questionnaire offers researchers, educators and service providers a tool to identifying shortcomings in staffs' knowledge, attitudes and practices. Having a baseline indication, will enable appropriate health education, health promotion initiatives and training programmes to be developed and evaluated