Simple Summary There are several rare types of hematologic malignancies and germline predispositions syndromes in children and adolescent young adults. In this descriptive review, we briefly describe rare hematologic malignancies, myelodysplastic neoplasms, and predispositions syndromes in children for which we have cohort outcome data and summarize emerging concepts in pathogenesis, diagnosis, prognostication, and treatment.Abstract There are a variety of rare hematologic malignancies and germline predispositions syndromes that occur in children and adolescent young adults (AYAs). These entities are important to recognize, as an accurate diagnosis is essential for risk assessment, prognostication, and treatment. This descriptive review summarizes rare hematologic malignancies, myelodysplastic neoplasms, and germline predispositions syndromes that occur in children and AYAs. We discuss the unique biology, characteristic genomic aberrations, rare presentations, diagnostic challenges, novel treatments, and outcomes associated with these rare entities.