This is a book for graduate students and researchers who wish to learn about carbon nanotubes and engage in the research and development of carbon anotubes and their applications in various fields.The carbon nanotubes were considered to be one of the most potential nanomaterials in this century. The field has recieved a considerable attention and is achieving continuos progresses in both research and industrial applications.The book includes a brief introduction on carbon nanotubes. But it mainly focuses on aligned carbon nanotubes. The synthesis and microwave absorption properties of aligned carbon nanotubes were investigated in the book. The all datas on aligned carbon nanotubes were obtained from a long time investigation in my group in past a dozen years. I hope the book can become a useful handbook for students and researchers who is engaging in studies and development of carbon nanotubes or will be in the field in the near future.