Participation inequality -the phenomenon that a very small percentage of participants contribute a very significant proportion of information to the total output -is persistent across Volunteered Geographic Information (VGI) and citizen science projects. It has been identified in both online and offline projects that rely on volunteers' effort over the past 20 years and, therefore, can be expected to appear in new projects. This chapter looks at participation inequality (also known as the 1% rule or the 90-9-1 rule), its origins and some of its characteristics. The chapter also explains how participation inequality emerges in a project at both temporal and spatial scales, and also evaluates its implication on the use of VGI and citizen science data. The chapter suggests a generic rule for analysts of VGI and citizen science datasets, in the form: 'When using and analysing crowdsourced information, consider the implications of participation inequality on the data and take them into account in the analysis. '