The primary objectives of the study are to determine the incidence of serious adverse events (SAEs) in participants with relapsing forms of multiple sclerosis (MS) in routine clinical practice and to assess the overall long-term clinical effectiveness of Plegridy in participants with relapsing forms of MS in routine clinical practice. The secondary objectives of this study in this study population are to describe Plegridy prescription and utilization adherence patterns in routine clinical practice; to assess the specific long-term clinical effectiveness of Plegridy in participants with relapsing forms of MS in routine clinical practice; to monitor the safety and tolerability of Plegridy in routine clinical practice by assessing the incidence of adverse events (AEs) of flu-like symptoms (FLS), injection site reactions (ISRs), and AEs (including laboratory abnormalities) leading to treatment discontinuation; to assess the effect of FLS on participant-reported effectiveness of, and satisfaction with, prophylactic management using a FLS-Visual Analog Scale (FLS-VAS); to evaluate the change in health-related quality of life (HRQoL), FLS, FLS-VAS, healthcare resource consumption, and treatment adherence over time.