The Swiss Interdisciplinary Longitudinal Study on the Oldest Old (Swilsoo) was designed to investigate life and health trajectories during advanced opld age and their repercussions on elders' daily life. This chapter, divided in two sections, first introduces the reader to Swilsoo's design, samples and fieldwork. Two cohorts were assessed on an approximately yearly basis, the first for nine waves of interviews from 1994 to 2004, with 340 participants initially, and the second for five waves from 1999 to 2004, with 377 participants initially. The starting sample of each cohort was stratified by gender, age and geographical area (urban versus semi-rural) and was composed of community-dwelling participants aged between 80 and 84 years; those who moved later to a nursing home remained in the study. Several aspects were assessed during the interviews (health, bereavement, social relationships and activities, well-being, cognitive ability, etc.). This chapter describes more specifically five main features of Swilsoo: 1) its 10-year longitudinal design; 2) its focus on the very old; 3) its dual focus on two birth cohorts; 4) the stratified and random selection of its participants; 5) its multidisciplinary and standardized closed-end questionnaire. A total of 2,890 questionnaires were gathered during the 10 years of the field work. The second section of the chapter gives an overview of the elders' life course through two world wars, the Great Depression, and later the Golden Decades (1946-1975); it then describes the survey population in terms of socioeconomic and cultural resources, family network and health, highlighting the main gender, geographical area and cohort differences.