for FM 3.1, and 2665 for FM 3.2), then considers ruptures of multiple segments regardless of which parent fault they belong to. After removing those ruptures considered implausible there are 253,706 possibilities to consider for FM 3.1, and 305,709 for FM 3.2. This compares to less than 8,000 ruptures considered in UCERF2, and reflects the high connectivity of California's fault system. A significant achievement of UCERF is development of system-level approach called the "grand inversion". This uses a supercomputer to solve a system of linear equations that simultaneously satisfies multiple constraints such as known slip rates, etc. The result is a model (set of values) that best fits the available data. In balancing these various factors it also provides an estimate of how much seismicity is not accounted for in the fault model, possibly in faults not yet discovered. The amount of slip occurring on unidentified faults has been estimated at between 5 and about 20 mm/yr depending on the location (generally higher in the LA area) and deformation model, with one model reaching 30 mm/yr just north of LA. Assessment While UCERF3 represents a considerable improvement over UCERF2, and the best available science to-date for estimating California's earthquake hazard, the authors caution that it remains an approximation of the natural system. There are a number of assumptions in the Time Independent model, while the final (Time Dependent) model explicitly "assumes elastic rebound dominates other known and suspected processes that are not included in the model." Among the known processes not included is spatiotemporal clustering. There are a number of sources of uncertainty, such as insufficient knowledge of fault geometry (especially at depth) and slip rates, and there is considerable challenge in how to balance the various elements of the model to achieve the best fit with the available observations. For example, there is difficulty fitting paleoseismic data and slip rates on the southern San Andreas Fault, resulting in estimates of seismicity that run about 25% less than seen in the paleoseismic data. The data does fit if a certain constraint (the regional Magnitude-Frequency Distribution) is relaxed, but this brings back the problem over-predicting moderate events. An important result is that the generally accepted Gutenberg-Richter (GR) relationship (that the distribution of earthquakes shows a certain relationship between magnitude and frequency) is inconsistent with certain parts of the current UCERF3 model. The model implies that achieving GR consistency would require certain