Following the era of the ‘golden age of marriage’ and the baby boom in the 1950s and 1960s, marriage has declined in importance, and its role as the main institution on which family relations are built has been eroded across Europe. Union formation most often takes place without a marriage. Family and living arrangements are currently heterogeneous across Europe, but all countries seem to be making the same shifts: towards fewer people living together as a couple, especially in marriage; an increased number of unmarried couples; more children born outside marriage; and fewer children living with their two parents. The relationship between these changing living arrangements, especially the decline of marriage, on the one hand, and the overall level of fertility, on the other, is not straightforward. In most countries, marriage rates and fertility declined simultaneously. However, the aggregate relationship between marriage and fertility indices has moved from negative (fewer marriages imply fewer births) to positive (fewer marriages imply more births). Thus, the decline of marriage, which is a part of the second demographic transition (see Overview Chapter 6), cannot be considered an important cause of the current low fertility level in many European countries. On the contrary, in European countries where the decline of marriage has been less pronounced, fertility levels are currently lower than in countries where new living arrangements have become most common.