Koen Joosten studied medicine at Radboud University Nijmegen. In 1993 he became a pediatrician and in 1995 he finished the follow-up pediatric intensivist at the Erasmus MC-Sophia Children's Hospital in Rotterdam, where he still works.
In 1999 he was promoted to the topic nutrition for the seriously ill child. His research focuses on both the nutritional screening for sick children and on optimal nutrition strategies for the seriously ill child. Together with Dr. Jessie Holly he developed the STRONG Kids screening tool to determine which children have an increased risk of malnutrition during hospitalization.