Interphase behavior of CNTs in a number of polymer systems is being investigated. These include the following: Polyacrylonitrile (PAN), poly(methyl methacrylate) (PMMA), polypropylene, and bismaleimide (BMI) resin. The studies include use of functionalized and un-functionalized CNTs. A variety of CNT dispersion methods have been used. Results show that the type of interphase has significant effect on the structure, rheology, crystallization behavior, and mechanical properties. For example due to PAN template formation on CNT, a graphitic layer is observed on CNT in PAN-CNT based carbon fibers. Helical wrapping of PMMA is observed on single wall carbon nanotubes. Three different types of interphases have been studied in PP-CNT system, showing different levels of influence on properties. Different types of interphase and interface is also being studied in BMI – CNT system. Results of these studies will be presented and discussed.