This paper reports on research carried out with SIPP to define households over time and construct associated measures of social and economic well-being on an annual basis.  The research, an extension of previous work with the 1979 Income Survey Development Program data, examines several alternative definitions of longitudinal households.  Measures of annual low-income status and household composition experience during the year are compared across definitions to shed empirical light on problems of longitudinal measurement for households, including problems of presenting data for part-year households and for longitudinal households that had some composition change during the year.