In April 2010, the UK Commission took over strategic ownership of the Investors in People (IIP) Standard. This evidence review draws on a wide range of research from the last 10 years to develop a deeper understanding of how IIP is perceived and to provide evidence of the impact of the Standard on the businesses which are accredited. The review provides the UK Commission with a consistent narrative on IIP to date. Over the past twenty years Investors in People has become an enduring feature of the UK skills policy landscape, as a training tool and as a business development tool for organisations. There is evidence of a continuing concentration of accreditations among larger organisations and employers in the public sector. The review identifies many reasons why employers engage with IIP. These differences in motivation are likely to influence the impact of IIP on organisations, which are commonly measured in the literature as relating to training levels, operational performance and business outcomes. There is a mixed understanding among employers and stakeholders as to whether IIP is a training tool, a business development tool, or both. The report raises interesting questions about the future objectives of IIP in encouraging more employers to invest in workforce capability.