Disorganized attachment in infancy is known to predict a wide range of maladaptive outcomes, but its origins are poorly understood. Parental lack of resolution concerning loss or trauma has been proposed to result in atypical parenting behaviors, which in turn have a disorganizing effect on the parent-child relationship. The authors review the evidence for this transmission pathway, considering other factors (e.g., social environment, child characteristics) that might enrich understanding of the antecedents of disorganization. A threshold approach is proposed to explain (a) why different parental behaviors are linked to disorganization depending on prevailing social conditions and (b) why certain children appear more vulnerable to forming a disorganized attachment relationship.