Fluctuating asymmetry (FA) is a measure of developmental stability, reflecting an organism's ability to produce an 'ideal' form under specific conditions. Bilateral structures in symmetrical organisms provide a reference for assessing deviations from symmetry. FA is commonly measured using frequency distributions of right-left (R-L) asymmetry, which can show three main patterns: FA, directional asymmetry (DA), and antisymmetry. FA is thought to increase under environmental or genetic stress and may correlate with fitness. However, it is assumed that asymmetry in individuals should not have a heritable basis, as this would complicate interpretations of developmental stability. Other forms of asymmetry, such as DA and antisymmetry, are less reliable as measures of developmental stability. Studies of FA often compare between-side variation across samples. Due to the small magnitude of asymmetry (typically <5% of trait size), precise measurement and analysis are crucial. Published FA data are difficult to compare due to varying methods and sample sizes. This primer aims to help researchers conduct FA analyses with maximum power and precision, avoid common pitfalls, and present data for quantitative comparison. It includes a checklist to guide FA studies. Key considerations include choosing appropriate traits, minimizing measurement error, testing for directional asymmetry, and accounting for size dependence of FA. Additionally, the relationship between asymmetry and fitness, as well as correlations among traits, are important aspects of FA research. The text emphasizes the need for careful analysis and interpretation to ensure accurate conclusions about developmental stability.Fluctuating asymmetry (FA) is a measure of developmental stability, reflecting an organism's ability to produce an 'ideal' form under specific conditions. Bilateral structures in symmetrical organisms provide a reference for assessing deviations from symmetry. FA is commonly measured using frequency distributions of right-left (R-L) asymmetry, which can show three main patterns: FA, directional asymmetry (DA), and antisymmetry. FA is thought to increase under environmental or genetic stress and may correlate with fitness. However, it is assumed that asymmetry in individuals should not have a heritable basis, as this would complicate interpretations of developmental stability. Other forms of asymmetry, such as DA and antisymmetry, are less reliable as measures of developmental stability. Studies of FA often compare between-side variation across samples. Due to the small magnitude of asymmetry (typically <5% of trait size), precise measurement and analysis are crucial. Published FA data are difficult to compare due to varying methods and sample sizes. This primer aims to help researchers conduct FA analyses with maximum power and precision, avoid common pitfalls, and present data for quantitative comparison. It includes a checklist to guide FA studies. Key considerations include choosing appropriate traits, minimizing measurement error, testing for directional asymmetry, and accounting for size dependence of FA. Additionally, the relationship between asymmetry and fitness, as well as correlations among traits, are important aspects of FA research. The text emphasizes the need for careful analysis and interpretation to ensure accurate conclusions about developmental stability.