Inland fisheries based on lakes and rivers are difficult to assess and data on catch and effort, often from subsistence fisheries, is lacking. Predictive models based on simple and multiple linear regressions enable estimates of fish yield that can be used in planning for management and development in the absence of catch and effort data. Based originally on physical, edaphic, hydrological and social data for African rivers, predictive relationships were refined and made more widely applicable by incorporating data from South America and Asia. For the lake assessments, additional variables were factored into the models, including climatic conditions (particularly rainfall). Such models provide only first order estimates but are valuable for planning.