Characterization of liquid hydrogen pooling has been a long-standing research focus since early experiments by Arthur D. Little/US Air Force in 1960. Since then, international research groups have been conducting experiments, seeking to understand how liquid hydrogen accumulates and pools on surfaces. Some of this work has also been aimed at developing relevant models to better assess safety, flammability, and explosion scenarios. Despite decades of effort, researchers are still formulating a better understanding of the vast variable space and related physical phenomena of liquid hydrogen spills and pooling. This presentation will convey historical work that has been completed to characterize liquid hydrogen pooling and the variables that are most significant. The influence of the probability of liquid hydrogen pooling on a horizontal surface, the variables involved, and why they are currently highly uncertain will also be discussed. More empirical data and better development of accurate models for pooling under highly variable conditions are needed to facilitate safe adoption of liquid hydrogen in various applications. Such understanding can significantly aid in assessing, identifying, and quantifying risk for liquid hydrogen use in a variety of industrial applications and locations, and assist in developing effective empirically-based safety codes and standards.