For a study, researchers sought to develop and test a comprehensive and user-friendly instrument for assessing the severity of total body ichthyosis in adults and children. Ichthyosis professionals were involved in the content development of the Ichthyosis Scoring System (ISS) in the qualitative study. The body was separated into 10 locations, and scale and erythema were quantified using Likert ratings (0-4) with specific descriptive and photographic standards. Photographs of individuals with ichthyosis were used to construct an 83-image instructional set. To assess interrater and intrarater reliabilities, 2 groups of dermatologists (a total of 11) independently assessed all test images twice. Participants were recruited from referral centres and patient advocacy groups worldwide. The ISS was created with participants of different ages, races, and ethnicities in mind, and dermatologists with varying levels of experience and expertise served as raters to assess the ISS. The research took place from 2019 to 2021, with the results being analysed in 2021. Overall reliabilities were assessed using intraclass correlation coefficients.  The intraclass correlation coefficients for total, scale, and erythema scores were greater than 0.90 (95% CI, 0.77-0.97), 0.91 (95% CI, 0.79-0.98), and 0.88 (95% CI, 0.72-0.97), respectively, across both cohorts of 11 dermatologists. Interrater reliabilities for scale and erythema were moderate to good in most body areas. Intrarater reliabilities ranged from acceptable to outstanding. The outcomes showed that the ISS was a reliable technique for assessing the severity of global ichthyosis in adults and children.