Owing to the recent classification of CDKL5 Deficiency Disorder (CDD) as a unique disorder, there is a limited understanding of overall disease natural history and meaningful outcome measures. An international patient registry aimed at collecting both patient/caregiver and clinician-entered demographic, patient-reported outcome (PRO) and treatment data would benefit both the scientific and patient communities. This CDD registry will follow up to 500 patients diagnosed with CDD over several years through both the patients/caregivers and their clinicians. Initial data will be collected upon enrollment in the registry, followed by the collection of additional CDD-specific data on a bi-annual/ annual basis. No procedures will be performed as part of this registry. Clinician-entered data will be collected following standard of care visits conducted as part of patients' ongoing clinical care. Ultimately, the goal is to create a contact registry to allow patients/families to be alerted about relevant clinical trials and to collect valuable information that is accessible to the patient and scientific communities, thereby aiding and encouraging research in CDD.