Since 1989, he has been working with injection drug users and conducting research on the contextual factors that influence HIV risk behaviors. In 1994, he published an article showing how the Colorado paraphernalia statute and policing strategies combined to increase HIV risk practices among injection drug users in Denver. In 2001–2002, Koester was a visiting Senior Behavioral Scientist in the Division of Viral Hepatitis at the Centers for Disease Control and Prevention, and in 2003 he began working on HIV and drug use in Bangladesh. In 1995 he was a Fulbright Research Scholar in St. Lucia. He received the University of Colorado at Denver’s Researcher of the Year award for 2003.