<p>California presents one of the most geotechnically challenging environments for transportation infrastructure in North America. The state’s seismically active setting, combined with its pronounced geological and climatic diversity, gives rise to soil conditions that vary considerably across short distances from saturated alluvial deposits along the coast to expansive clays in the interior valleys and colluvial soils on steep mountain slopes. Yet despite the breadth of site-specific studies on hazards such as liquefaction, differential settlement, and slope failure, a synthesis that connects regional soil variability, hazard mechanics, ground improvement practice, and data-driven design within a single framework has been largely absent from the literature. This paper attempts to address that gap. The review covers the engineering properties and spatial distribution of major California soil types expansive clays, alluvial and estuarine deposits, residual soils, and coastal sediments and traces how each responds to seismic loading. Cyclic soil behavior, liquefaction susceptibility, lateral spreading, and soil–structure interaction are examined in the context of roads, bridges, embankments, and port infrastructure. Foundation distress arising from shrink–swell cycles in semi-arid regions is also considered, alongside erosion and instability risks along vulnerable transportation corridors. The literature search drew on Scopus, Web of Science, Google Scholar, and USGS technical publications. Both established stabilization methods compaction, geosynthetics, lime and cement treatment and newer approaches such as geopolymer binders and bio-mediated precipitation are evaluated for their applicability in seismic contexts. The paper argues that durable, low-carbon ground improvement, guided by site-specific subsurface data and probabilistic hazard assessment, is essential to the long-term reliability of California’s transportation network.</p>