We present an approach for personalizing nonpharmacological interventions for people with dementia (PwD) using ontologies. We conducted two case studies to derive an ontological model to personalize the planning and execution of interventions to address problematic behaviors. The paper describes how the ontology was derived, and illustrates how it is used to tailor an ambient-assisted intervention system (AAIS) at two stages: first, to decide on the services that the AAIS will offer the PwD, and then to adapt these services at runtime using contextual information. The results of a deployment of an AAIS during 2 months in the home of a PwD, indicate that the AAIS successfully addressed some of the problematic behaviors exhibited by the PwD, helping to reduce the burden of the caregiver.