  We study the extension problem for quasidiagonal (QD) C*-algebras (i.e. when
is an extension of QD C*-algebras again QD?). The main positive result states
that in many instances an extension will remain QD provided that a certain
boundary arising from the K-theory of the extension vanishes. A K-theoretic
Hahn-Banach type property is also introduced for QD C*-algebras. We show that
every nuclear QD C*-algebra has this Hahn-Banach property if and only if the
extension problem for many QD C*-algebras is completely determined by a
boundary map on K-theory.
