How large can a set in a finite abelian group G be, given that it does not contain three elements in an arithmetic progression? If G is the additive group of the vector space 𝔽3n, then three-term progressions are lines, and we are asking about the largest size of a line-free set in 𝔽3n. We review the recent solution of this problem due to Ellenberg and Gijswijt, and discuss related open problems.