This report investigates what makes a ‘good’ joint strategic needs assessment (JSNA) for mental health or dementia that can bring about real change in people’s lives. It looks at how five local areas across England carried out their JSNAs to understand how they produced their needs assessments, how they used them and what impact they had. The local areas visited were: Blackburn with Darwen, Bradford, Haringey, Surrey, and Swindon. The analysis found that needs assessments can help raise the profile of mental health and dementia, direct investment, improve services and help local agencies work together more effectively. The report highlights eight factors that characterise successful needs assessments for mental health and dementia that result in the greatest impact. These are: effective leadership to ensure JSNA findings result in action once the assessment is completed; a clear purpose; engagement and collaboration when creating the statement, national policy directives; reliable and up-to-date evidence and data; including the views of people with lived experience, presentation, ensuring the findings are easily accessible; and follow up. To have the biggest impact, they need a clear purpose, effective leadership and advocacy, and partnerships that continue after the JSNA is completed to ensure that they lead to action. The report also provides key messages for local and national bodies.