Whose vs Who’s: Possession or “Who Is”?

Whose shows possession. Who’s is a contraction of who is or who has. The apostrophe does not mark possession here.

Whose laptop is this? asks about ownership. Who’s presenting first? expands to Who is presenting first?

The substitution test

Replace who’s with who is or who has. If neither works, you probably need whose: The researcher whose paper we discussed…

For the wider grammar, see who, whom and whose. Possession more generally is covered in English possessives.

Why the apostrophe misleads learners

English often uses an apostrophe for possession with ordinary nouns, as in the teacher’s notes. Pronouns follow a different system: whose, his, hers, ours, theirs have no possessive apostrophe. That is why visual similarity is a poor guide here; grammatical function is the better guide.