Critical.
(Note that documentation can be electronic, not necessarily paper.)
• QA practices should be documented such that they are repeatable.
• Specifications, designs, business rules, inspection reports, configurations, code changes, test plans, test cases, bug reports, user manuals, etc. should all be documented.
• There should ideally be a system for easily finding and obtaining documents and determining what documentation will have a particular piece of information.
• Change management for documentation should be used if possible.