Purpose <p>CDKL5 deficiency disorder (CDD) is a rare developmental and epileptic encephalopathy. Greater understanding of the smallest meaningful improvements for individuals with CDD in clinical trials and practice is needed for a person-centred approach to treatment efficacy. This study explored how parent/caregivers of people with CDD understood meaningful improvements and described change for priority functional domains including communication, gross motor, fine motor, feeding.</p> Methods <p>This study included an in-person workshop and a convergent mixed-methods online survey. Parent/caregivers (<i>n</i> = 19) attending the 6th Family Educational and Awareness Conference for CDD participated in discussion groups for the workshop component. The survey (<i>n</i> = 80) collected descriptive data and open-ended responses. Qualitative data were stratified by ability levels and analysed using a conventional content analysis.</p> Results <p>Definitions of meaningful improvement varied in terms of desired speed and magnitude of change and included health and skill stability. Parent/caregivers described meaningful increases in developmental skills that were specific to each domain and level of ability. Some concepts were common across ability levels within domains (e.g., increased independence across all gross motor levels) and others were consistent across domains (e.g., improved utensil use in fine motor and feeding domains).</p> Conclusion <p>Meaningful improvement means different things to different people with some factors consistent regardless of ability level suggesting important underlying concepts for measurement requiring future investigation. These findings can contribute to the development of clinical treatments and trials that focus on factors that are important to people with CDD and their families.</p>