Hotspots are regions of voluminous volcanism, such as Hawaii and Iceland. However, direct tests by mid-mantle tomography and petrology of lavas do not conclusively resolve that mantle plumes and mantle high temperature cause these features. Indirect and model-dependent methods thus provide constraints on the properties of plumes, if they exist. Classical estimates of the excess (above mid-ocean ridge basalt values) potential temperature of the upwelling mantle (before melting commences) are 200–300 K. The estimate for Iceland utilizes the thickness of the oceanic crust. Estimates for Hawaii depend on the melting depth beneath old lithosphere. Flux estimates for Iceland depend on the kinematics of the ridge and those of Hawaii or the kinematics and dynamics of the Hawaiian swell. These techniques let one compute the global volume and heat flux of plumes. The flux is significant—approximately one-third of the mantle has cycled through plumes, extrapolating the current rate over geological time. However, obvious vertical tectonics may not be evident for many hotspots. For example, the relatively weak Icelandic hotspot (one-sixth of the volume flux of Hawaii) would produce only modest volcanism and swell uplift if it impinged on the fast-moving Pacific plate or the fast-spreading East Pacific Rise.