This study provides a historical review, a meta-analysis, and recommendations for the proper use of weight-length relationships, condition factors, and relative weight equations in fisheries research. The historical review traces the development of these concepts, from Galileo's cube law to modern allometric growth models. The meta-analysis examines 3929 weight-length relationships for 1773 fish species, revealing that 82% of the variance in the relationship between the intercept ($a$) and the exponent ($b$) can be explained by allometric versus isometric growth patterns and body shape differences. The median value of $b$ is significantly larger than 3.0, indicating a tendency towards slightly positive allometric growth in most fish species. The expected range of $2.5 < b < 3.5$ is confirmed, with mean estimates outside this range often based on limited data. True cases of strong allometric growth are rare, but three examples are provided. Within species, a plot of $\log a$ vs $b$ can help detect outliers in weight-length relationships. The study also provides equations for calculating mean condition factors and relative weight, and offers twelve recommendations for the proper use and presentation of these data.This study provides a historical review, a meta-analysis, and recommendations for the proper use of weight-length relationships, condition factors, and relative weight equations in fisheries research. The historical review traces the development of these concepts, from Galileo's cube law to modern allometric growth models. The meta-analysis examines 3929 weight-length relationships for 1773 fish species, revealing that 82% of the variance in the relationship between the intercept ($a$) and the exponent ($b$) can be explained by allometric versus isometric growth patterns and body shape differences. The median value of $b$ is significantly larger than 3.0, indicating a tendency towards slightly positive allometric growth in most fish species. The expected range of $2.5 < b < 3.5$ is confirmed, with mean estimates outside this range often based on limited data. True cases of strong allometric growth are rare, but three examples are provided. Within species, a plot of $\log a$ vs $b$ can help detect outliers in weight-length relationships. The study also provides equations for calculating mean condition factors and relative weight, and offers twelve recommendations for the proper use and presentation of these data.