Risk networks can transmit HIV or otherinfections; social networks can transmit social influenceand thus help shape norms and behaviors. This primarily-theoretical paper starts with a review of networkconcepts, and then presents data from a NewYork network study to study patterns of sexual andinjection linkages among IDUs and other drug usersand nonusers, men who have sex with men, womenwho have sex with women, other men and other womenin a high-risk community and the distribution ofHIV, sex at group sex events, and health intraventionbehaviors in this network. It then discusses how risknetwork microstructures might influence HIV epidemicsand urban vulnerability to epidemics; whatsocial and other forces (such as ‘‘Big Events’’ like warsor ecological disasters) might shape networks and theirassociated norms, intraventions, practices and behaviors;and how network theory and research have andmay continue to contribute to developing interventionsagainst HIV epidemics.