The purpose of this observational study is to model factors that determine physician treatment decisions in selecting specific second line agents in pediatric ITP and to determine the comparative effectiveness of second line ITP treatments by bleeding measures, platelet counts, and patient reported outcome measures. This prospective observational, longitudinal, multicenter cohort study will aim to collect routine clinical care data, quality of life information from patients, and decision making data from clinicians at enrollment and at regular clinical intervals for at least one year. The primary and secondary objectives are as follows: