Recent studies demonstrate that sexual minority youth are at higher risk for mental health problems than heterosexual youth. My dissertation seeks to explain that finding by examining and comparing the parent and peer relationships of sexual minority youth and heterosexual youth. I hypothesize that sexual minority youth receive less social support from their parents and peers than heterosexual youth, which contributes to a greater risk of mental health problems for sexual minority adolescents. Using statistical methods in a nationally-representative dataset, I find that sexual minority youth have less social support from parents than heterosexual youth, which in turn increases risk for mental health problems among sexual minority youth. In examining the peer relationships of sexual minority youth, I find some evidence that sexual minority youth are less likely to have friends and are less popular in peer networks than heterosexual youth. Yet, the peer relationships of sexual minority youth did not contribute to the risk of mental health problems among sexual minority youth. In conclusion, sexual minority youth have more disadvantaged relationships with parents and peers than heterosexual youth, though the relationships with parents contribute more to the increased risk of mental health problems among sexual minority youth than the relationships with peers.