Background: Multiple sclerosis (MS) is a chronic progressive disease, affecting a broad range of important functions. In order to accurately measure disease severity among people living with MS (PLwMS), and provide the best care, it is important to understand which domains are most bothersome to patients. Such information may also inform the development of digital approaches to measuring clinically relevant aspects of MS. Objectives: To gain insight into the most common and bothersome symptoms and impacts of MS across disease subtypes and disability levels, via interviews with expert clinicians and PLwMS. Methods: Semi-structured interviews were conducted with clinical experts (neurologists who treat PLwMS) and PLwMS diagnosed with relapsing-remitting MS (RRMS), secondary progressive MS (SPMS) or primary progressive MS (PPMS). Participants completed the MS-specific patient-reported Expanded Disability Status Scale (prEDSS) and Patient-Determined Disease Steps to determine their disease stage and disability level. Sixty-minute 1:1 telephone interviews were conducted with study participants to gain insight into the frequency and importance of symptoms and impacts of MS, with a specific focus on upper limb function, lower limb function, cognition and vision. Interviews informed the development of a disease conceptual model as well as conceptual frameworks for each domain of interest. Results: A total of 40 PLwMS (female 62.5%; prEDSS mean score 5.0 [range: 28]; mean age 52.4 years [range: 2868]), including participants with RRMS (n=12), SPMS (n=8), and PPMS (n=20), were interviewed, as well as five clinical experts (mean 19.2 years of experience [range: 243]). Clinical experts reported consistent symptomatology across the MS subtypes, but with differences according to disease stage. Likewise, PLwMS described similar experiences and impacts of MS, regardless of subtype, with fatigue, cognitive impairment, sensory issues, mobility issues, hand function, bladder dysfunction, and vision impairment most frequently reported. Fatigue and mobility issues were the most bothersome symptoms. Conclusions: This study was the first phase of a two-part qualitative study to identify key MS symptoms and functional domains as reported by clinical experts and PLwMS of all subtypes. These results can be used to support the concepts and scoring approaches for digital tools such as FloodlightTM MS by ensuring that the most pertinent functional aspects of MS are assessed.