Families exist everywhere in the world to bear and rear children, to care for and protect vulnerable members during old age, illness, and misfortune, and to meet our relational needs. They are the bedrock of cultural traditions, norms and values, and behaviour patterns transmitted from one generation to the next. Our whole lives are spent in close interaction with family members and, as such, families play a critical role in our mental and physical well-being at all ages, but especially in the formative development period of childhood and adolescence. We begin here by describing the various forms that families can take, the central role families play in children's health and well-being, the ways in which dysfunction in the family can have long-lasting effects on mental health and well-being, and what is necessary to support families to best provide nurturing care. While families are universal, they are not uniform with respect to size, gender, age groups or even whether they are based on biological or social connections. Together, these characteristics define family form - the shape, composition and structure of a family unit. For the purposes of this chapter, we consider a family to be a single individual or group of individuals related to each other either socially or biologically, with at least one child in the family unit. Globally, families are becoming smaller with fewer children, generally leading to more investment in the health and education of each individual child, a long-standing trend beginning in the late 1800s described in Viviana's Zelizer's book, Pricing the Priceless Child: The Social Values of Children.