Generalizability Theory (G-Theory) represents a powerful statistical framework that revolutionizes reliability assessment by addressing the multifaceted nature of measurement error. Unlike traditional approaches which typically produce a single reliability coefficient, G-Theory systematically deconstructs and quantifies multiple sources of variation simultaneously—whether from raters, items, occasions, or other measurement facets. This sophisticated analytical approach provides researchers with unprecedented insights into the dependability of their measurements across diverse contexts and applications. GeneralizIT brings this advanced methodology to the Python ecosystem through an intuitive, research-oriented library. The package seamlessly handles complex research designs (crossed and nested), accommodates real-world data challenges (unbalanced data and missing observations), and delivers comprehensive analytical outputs including variance component estimation, generalizability coefficients, and decision studies (D-studies) that enable researchers to optimize measurement protocols with precision and confidence.