This chapter reviews the clinical events that occur in patients with juvenile-onset spondyloarthritides (SpA) with the purpose of developing core sets, domains and instruments to evaluate disease activity and disease damage. We discuss several aspects, from concept and classification to clinical features and instruments already in use for measuring adult-onset SpA and childhood arthritides. Similarly, comparisons between juvenile-onset SpA, its adult counterpart, and other forms of juvenile arthritis are made to consider the adaptation of existing instruments or to develop specific ones.