Insights from half a century investigating seafloor hydrothermal circulation on Earth can inform exploration on other Ocean Worlds. Early data/models for Earth provided useful predictions for flow at rift axes, but interpretations of the distribution of subseafloor circulation were revised significantly as data and simulations improved. Lessons learned for Earth systems elucidate how investigators might assess hydrothermal processes on other Ocean Worlds. Basin-scale morphology and heat flow indicate whether conduction explains the flux or cooling by seawater advection occurs. Detailed sonar mapping reveals areas where high flux is most likely. Seismicity patterns suggest different modes of circulation: East Pacific circulation is mapped via microseismicity within porous basaltic crust; Regional seismicity in the Atlantic highlights detachment-dominated rift segments, where faults channel circulation, sometimes within peridotite blocks that react with seawater. Exothermic serpentinization, also inferred for the lower ocean crust near subduction zones, should impart detectable temperature (T) signals. High-T/-flux circulation may be recognized with remote sensing (sonar, thermistors, chemical sensors, cameras) at the seafloor whereas low-T/-flux systems are difficult to identify. Modeling examines factors that influence hydrothermal circulation. Early models were simple single-pass cases; later data and more complex models show circulation includes multi-pass convection that can be 3D, unstable, and strongly guided by permeability patterns. Borehole observatories reveal heterogeneity in fluid chemistry and crustal microbiology. Interpreting Ocean World hydrothermal conditions will be challenging, but some fundamental inferences for Earth’s seafloor hydrothermal systems have proven robust. Early studies generated testable hypotheses, advanced instrumentation and simulations, and helped focus interdisciplinary discovery.