In Britain around 1900, established financial institutions for long-term savings such as life assurers, and pension funds which were just in their formative phase, did not make material allocations to publicly quoted equity markets or ordinary shares; long-established life assurers, for example, had less than 3 per cent allocated to the asset class (Baker and Collins 2003). Over the following 100 years, this picture radically changed, with equities emerging as the central asset class for many institutional investors and the term ‘the cult of (the) equity’ was coined (Scott 2002; Avrahampour 2015). As the century progressed, institutional investors superseded private individuals and became the dominant holders of British publicly quoted companies (Cheffins 2010). Despite the attractions of the asset class and their generally high returns, within a relatively short period by the end of the century, institutional equity exposure had peaked and was in decline both at life assurers and within pension funds. Here we highlight, and link together, the key actuarial (Turnbull 2017) and investing (Morecroft 2017) ideas that were influential in these developments. We also identify the main individuals who were instrumental in the application of equity investing to institutional portfolios. The article has an emphasis towards years from 1920 to 1960 when most of the changes to investment practice and actuarial theory occurred.