When you need one
- Redesigns and migrations. New sites usually have new URLs. Every old address that had traffic or links needs a 301 to its closest new equivalent, or those visitors hit a 404 and the rankings are lost. This is the single most common way redesigns damage SEO.
- Renaming or deleting a page. Redirect the old URL to the new or most relevant page.
- Merging duplicate pages. Two pages on the same topic split their signals; redirect the weaker to the stronger.
- Changing domains, or moving from http to https, or from www to non-www. Every variant should redirect to the one canonical form.
301 versus 302 and other codes
A 301 says the move is permanent, so search engines update their index to the new address. A 302 says temporary, so they keep the old address indexed and expect it back. Use 301 for anything that is not coming back. A 404 says the page does not exist and a 410 says it was deliberately removed; both are fine for pages that genuinely have no replacement, as explained in what a 404 error means. Redirect chains (A to B to C) and loops slow pages down and should be flattened to A to C.
How they are set up
On WordPress, a redirect plugin or the SEO plugin's redirect manager; on Shopify, 'URL redirects' in the admin; on custom and Next.js sites, in the site configuration or the server. For a redesign, the developer should produce a redirect map before launch: a list of every old URL and where it goes, built from the old sitemap and from the pages Search Console shows getting traffic. Checking that the map works is part of the launch, and checking for lingering 404s is part of website maintenance. The website redesign page describes how we protect rankings through a rebuild.