The present study examined the factors influencing childcare workers' promotion of physical activity (PA) among children aged 0-4 years, a particularly interesting context because of the increasing number of children attending childcare. Twenty Dutch childcare workers were interviewed. The interviews revealed some important barriers to the promotion of children's PA: lack of facilities and time; rules and policies regarding safety and daily duties; non-cooperative colleagues; a large range of ages of children; poor weather; unsuitable clothing and lack of communication with parents. The respondents identified the parents rather than themselves as persons responsible for promoting children's PA. Parents and childcare workers could improve their communication regarding children's PA. Childcare centres could review their internal policies and provide workshops teaching childcare workers how to promote children's PA. (author abstract)