This chapter explores the representation of marriage in Robert Lowell’s poetry. It considers the often-controversial relationship between his biography and the published poems, and further explores how he uses marital dynamics to interrogate knowledge of self and other. At various times throughout his career, Lowell examines the social and domestic structures provided by marriage, representing the institution as a space of stability and emotional intensity, and therefore one that both inspires and supports creative achievement. Lowell’s third union, with Caroline Blackwood, intensified his engagement with the subject of marriage, prompting deeper analysis of states of intimacy and endurance in this final phase of his writing life. This chapter begins with Lowell’s late work, in particular with the volume Day by Day (1977), but argues that his preoccupation with marriage can be traced back through The Dolphin (1973) to the confessional mode of the era-defining Life Studies (1959) and to its radical approach to tone and form that, at that time, enabled Lowell to interrogate the vexed relationship between private and public histories in profound ways.