Background: Parents' tacit knowledge plays an important role in the care of persons with profound intellectual and multiple disabilities (PIMD). As little is known about its nature and use, we aimed to explore this parental tacit knowledge. Method: We conducted semi-structured interviews with parents (n = 11) about their tacit knowledge of their child, based upon video recordings they made of their child's behaviour, and analysed the data thematically. Results: Parents described their tacit knowledge as the capacity to read their child's subtle signs, or to sense and "just know" their child's situation. They had developed this knowledge because of their shared history of proximity and interaction and emphasised that it was crucial in ensuring their children's needs are met. Conclusions: We describe how parents' tacit knowledge contributes to "good care" for persons with PIMD, interpret the implications for (medical) care practice, and discuss ways to deal with its limitations.