  We perform a review of various approaches to the implementation of quantum
memories, with an emphasis on activities within the quantum memory sub-project
of the EU Integrated Project "Qubit Applications". We begin with a brief
overview over different applications for quantum memories and different types
of quantum memories. We discuss the most important criteria for assessing
quantum memory performance and the most important physical requirements. Then
we review the different approaches represented in "Qubit Applications" in some
detail. They include solid-state atomic ensembles, NV centers, quantum dots,
single atoms, atomic gases and optical phonons in diamond. We compare the
different approaches using the discussed criteria.
