This thesis reviews recent trends in partnership formation and dissolution in Britain before investigating, for one particular birth cohort, the individual level determinants of partnership formation and dissolution. The work uses data from the National Child Development Study which has followed up since birth those born in Britain in one week of March 1958. We compare data from the age 33 partnership histories with those collected at age 23. We find that dates of entry into cohabitation are reported less accurately than for marriage and that the reliability of reporting decreases for men, those with less education and those with more complex partnership histories. Implications of these findings for future data collection are discussed. Taking a lifecourse perspective and the methodology of discrete-time logistic regression hazards models we examine the family background and current lifecourse factors influencing the timing and type of first partnership formation, the outcome of cohabiting first partnerships, and the role of premarital cohabitation and other factors on the risk of first marriage, dissolution. Socio-economic factors were found to be most important in determining the age at which partnership formation takes place, and whether partnership formation follows a premarital pregnancy. Attitudinal and cultural factors appear to influence the decision whether to marry directly or cohabit. For example cohabitation was more common among men and women with lower levels of religiosity, those whose parents separated, and among those brought up in the South and South East of Britain. Among this cohort entry into cohabitation was particularly likely among those living independently of the parental home: a finding of particular significance given the recent trends towards increased non-family living in young adulthood in Britain. By using a lifecourse approach we demonstrate that many socio-economic and family background factors influence marital instability through more intermediate, demographic covariates, such as age at marriage and the timing of childbearing.