Based on the reference text, provide a bulleted list of the three stages of ISO 27001 Certification and the activities of each stage.

The following are the three stages of ISO 27001 certification:
1. Stage 1: A preliminary, informal review of the Information Security Management System. Activities include checking for existence and completeness of primary documentation.
2. Stage 2: A detailed and formal audit performed by a third party to validate adequate implementation of the requirements specified in ISO 27001
3. Ongoing: Follow-up reviews and audits to confirm continued compliance with the requirements of ISO 27001.