Abstract Parenting programs have strong evidence in promoting positive changes in both parents and children and are considered as evidence-based treatments for children with externalising problems such as conduct problems and ADHD. It is expected that a public health approach to parenting - i.e., the availability of parenting programs to all parents regardless of their risk status - will reduce the number of children with externalising problems at the population level. However, the evidence on the effectiveness of this approach is inconclusive. This is largely due to difficulties in conducting the needed large population trials of universal parenting programs. The objective of the workshop is to facilitate a discussion on planning and conducting research projects to successfully evaluate the effectiveness of psychological interventions such as parenting programs when they are offered universally to everyone. We will describe three main challenges in evaluating a public health approach to parenting: difficulties in collecting data from the whole population (and not only those exposed to the intervention), lack of validated instruments to measure population-level impact, and low program exposure. To illustrate our points, we use a population-based cluster randomised controlled trial as an example. The aim of the project - Children and Parents in Focus - was to test the effectiveness of a universally offered parenting program in addressing emotional and behavioural problems in preschool children. The project was carried out for four years in a municipality with 200 000 inhabitants in Sweden. Data were collected from mothers, fathers and teachers of over 7 000 children aged 3 to 5 years. The workshop will include a brief introduction and three presentations. In the first presentation, we focus on recruitment of parents into the study. We discuss the rational for recruiting parents of all children aged 3-5 into the study through child health centres and the steps that we took to increase the number of parents who participated in the study. In the second presentation, we describe the primary outcome measure in the study i.e., the Strengths and Difficulties Questionnaires (SDQ) collected from fathers, mothers and preschool teachers. We discuss how we validated the SDQ for all the three informants and developed a guideline for using the SDQ during routine health check-ups. In the third presentation, we explain our challenges in recruiting parents to the intervention, particularly the restrictions imposed on us by the trial design and describe how we addressed low program participation. After each presentation, the participants will be invited to share their experiences, thoughts and ideas. Their comments will be first collected through a digital interactive platform and then discussed in the room. Key messages Successful evaluation of a public health interventions at the population-level requires ample attention to the recruitment of participants into research and intervention. Program uptake can be increased using simple low cost direct-to-consumer marketing strategies; however, it does take time to build demand for the program.