Coherent population trapping in open and closed systems

In the present paper, an overview on work concerning coherent population tr apping within the hyperfine manifold of the sodium D-1-Line is given. Besid es manifestation of coherent population trapping for different Lambda-schem es, also phase dependent processes in closed contour systems (four-level do uble Lambda-schemes) are discussed, including the case of phase dependent e lectromagnetically induced transparency.