Cert number lookup
Type the number off the slab. You get the coin, the grade, how many exist at that grade and what the guide says it is worth today.
PCGS certification numbers only. NGC-slabbed coins aren't supported yet — entering an NGC cert number may return an unrelated PCGS coin instead of an error, since PCGS and NGC number their certs independently.
PCGS numbers only
PCGS and NGC number their holders independently, and the two sets overlap. An NGC number typed in here will not error. It will quietly return whichever PCGS coin happens to share that number, which is worse. Check the holder before you trust the result.
Browsing rather than looking something up? Start from the coin types.