  This article is an expanded version of the author's lecture in the Basic
Notions Seminar at Harvard, September 2013. Our goal is a brief and
introductory exposition of aspects of two topics in sieve theory which have
received attention recently: (1) the spectacular work of Yitang Zhang, under
the title "Level of Distribution," and (2) the so-called "Affine Sieve,"
introduced by Bourgain-Gamburd-Sarnak.
