People walk a lot—to walk pets, to exercise and recreate, and to access public transit and local shops. Walk trips begin and end almost every journey, even trips made by automobile. Data from the current California Household Travel Survey (CHTS) show that walking occurs more than trips by both transit and bicycle, making it the second most common travel mode in California. Yet outside of select case studies in specific metropolitan areas, we know very little about walking behavior in California. An improved understanding of the determinants of walking will aid efforts to reduce driving and achieve greenhouse gas emission reduction targets. In this study we draw on data from the last two California Household Travel Surveys to examine walking behavior in four major California regions—the San Francisco Bay Area, Los Angeles, Sacramento, and San Diego. The study includes four components; analyses of(a) the change in walking over time (b) the relationship between walking and the built environment (c) the determinants of change in walking over time and (d) the relationship between changes in neighborhood characteristics and changes in walking. In each of the analyses, we pay particular attention to differences across these four metropolitan regions. We pair our statistical analysis with a set of interviews intended to understand whether and how walking trips are included in regional travel demand models. We find that, although walking remains a relatively small share (9%) of trips within the study area, walking rates have increased dramatically over time. The share of trips by walking grew almost twofold since 2001; from 5 percent to 9 percent. Moreover, while the share of walking trips is relatively small, walking mode shares are nine times higher than the percentage of trips taken by public transit or bicycle.