This analysis uses qualitative interview data from the Time, Love and Cash in Couples with Children study to explore the dimensions and evolution of relationship quality for parents who were unmarried when their child was born but still together four years later. I compared relationship quality over time for parents who went on to marry or were still together four years after their child’s birth. I found couples who married had higher levels of relationship quality at baseline and these differences persisted over time. Couples that married exhibit more supportiveness, love and trust in each other and experienced fewer serious problems such as infidelity, domestic violence, substance abuse and illegal activities or jail than the other couples. They also exhibited several positive trends in relationship quality over time. Couples that did not marry, but remained together comprised a heterogeneous group with varying levels of relationship quality, although the majority experienced poor or declining relationship quality over time. I discuss implications of the findings for the relationship between relationship quality and union formation for unmarried parents.