The investigators want to understand more about what kinds of health problems are caused by histiocytosis, what happens as a result of different treatments, and how ECD affects people's lives, their feelings and their attitudes. The investigators also want to learn how these things change over time for people with these conditions. To try to figure this out, doctors would like to collect information about people who have histiocytosis and how they are treated for this disease.

Some participants will be asked to complete a brief interview over the telephone. Only a limited number of interviews will take place, and not everyone will be asked to participate in the interview. It is okay if the participant does not want to participate in the interview portion. If this is the case, the participant should let the study team member know when contacted.

Inclusion Criteria:

• Diagnosis of Erdheim-Chester Disease, Langerhans cell histiocytosis, Rosai Dorfman disease, or another histiocytic neoplasm by either 1) documented histopathologic establishing ECD or HN or 2) documented histopathologic findings compatible with Erdheim-Chester disease or HN in the context of corroborating clinical and/or radiologic findings or 3) documented radiologic findings, in the absence of a biopsy having been performed or yielding evaluable tissue, that are felt by the Principal Investigator to unequivocally represent an ECD/HN diagnosis given the clinical context or 4) self reported with supporting documentation upon medical record collection.
• Proficiency in English, in the determination of the Investigator or by self report.
• Willing to have historical and future HN-related health records sent to Registry review.

Exclusion Criteria:

• Patients unwilling to sign consent.
• Participants under the age of 18.

Patients with Erdheim-Chester Disease and other HN will be enrolled in this registry study.