The First 100 Days on the Job is for sustainability leaders – in organizations of any size or sector – who want to make an impact in their first one hundred days on the job, and set themselves up for long-term success. In the absence of complete and perfect information you will be expected to lead and to act, often in partnership with other businesses, government and civil society, and almost certainly by building relationships across functions, departments and geographies within your own organization. It is the timing of your decision making that will set you apart.This short guide offers: 1) A process to make the most of your first 10, 60, 90 and 100 days 2) Some practical tools you can use to set priorities and manage your programme 3) Sources of research and information for measuring the impacts of your organization 4) A heavy dose of realism about what can be done, to keep you sane – and links to some practical support and inspiration.