The next generation fiber optical communication systems can be characterized in three trends: higher speed, better spectrum efficiency and longer transmission reach. To meet the requirement of next generation systems, passive DWDM components must have the following characteristics, low dispersion, small channel spacing, low insertion loss and high isolation. In this paper, we discuss one of the most important passive components: interleaver. Interleavers have been widely used in fiber optical communications systems. Their main functions include mux, demux, optical add/drop and bi-directional noise reduction. As data rate of fiber optical communication systems increases to 10Gb/s or higher, chromatic dispersion (CD) of interleavers becomes increasingly important. It has been shown that chromatic dispersion of interleavers could significantly limit the transmission reach. In this paper, we discuss 25GHz and 50GHz interleavers and their applications. Chromatic dispersion of several interleavers available on the market are discussed. We explain the generic relation between CD and channel spacing. The CD and CD slope of some interleavers can be reduced or completely compensated by innovative designs. We present an interleaver design with extremely low CD and CD slope. Finally, we present a 10Gb/s 160-channel transport application by using the low-CD 25GHz and 50GHz interleavers. The eye diagram and bit error rate of the 10Gb/s system will be discussed.