The　Tower　of　Pisa　survived　several　strong　earthquakes　undamaged　over　the　last　650　years,　despite　its　leaning　and　limited　strength　and　ductility.　No　credible　explanation　for　its　remarkable　seismic　performance　exists　to　date.　A　reassessment　of　this　unique　case　history　in　light　of　new　seismological,　geological,　structural,　and　geotechnical　information　is　reported,　aiming　to　address　this　question.　The　following　topics　are　discussed:　(1)　dynamic　structural　identification　based　on　recorded　earthquake　data;　(2)　geophysical　site　characterization　using　a　two-dimensional　array;　(3)　seismic　hazard　and　site　response　analysis　considering　horizontal　and　vertical　motions;　and　(4)　soil-structure　interaction　(SSI)　analysis　calibrated　using　lab　and　field　data.　A　substantial　shift　in　natural　period,　from　about　0.35　s　to　over　1　s　(a　threefold　increase,　the　largest　known　for　a　building　of　that　height)　caused　by　SSI,　a　wave　parameter　(1/sigma)　of　about　0.3,　and　a　minor　effect　of　vertical　ground　motion　are　identified　and　may　explain　the　lack　of　earthquake　damage　on　the　Tower.　Recommendations　for　future　research,　including　the　need　to　establish　a　seismic　bedrock　deeper　than　500　m,　are　provided.