Since the principle of multi-carrier code division multiple access (MC-CDMA) was simultaneously proposed by Khaled Fazel et al. and Nathan Yee et al. at the IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC) in the year 1993, multi-carrier spread spectrum (MC-SS) has rapidly become one of the most wide spread independent research topics on the field of mobile radio communications. Therefore, the International Workshop on Multi-Carrier Spread Spectrum (MC-SS) was initiated in the year 1997. Multi-carrier and spread spectrum systems with their generic air interface and adaptive technologies are considered as potential candidates to fulfill the requirements of next generation mobile communications systems. The material summarized in this volume was selected for the 6th International Workshop on Multi-Carrier Spread Spectrum (MC-SS 2007) held in Herrsching, Germany from 07-09 May 2007. The workshop was organized by the Institute of Communications and Navigation of the German Aerospace Center (DLR) in Oberpfaffenhofen, Germany. During the workshop, many hot topics in the field of MC-SS were covered. First, several presentations regarding General Issues were given. Iterative Decoding strategies were also discussed. Furthermore, a special session was addressed on Interleave Multiple Division Access (IDMA). In the areas of Spectrum and Interference, Data Transmission, Adaptive Transmission, Multiple Antennas, Multi-Cell and Multi-Link, and Synchronization and Tracking new and innovative algorithms, strategies, and systems were proposed. Finally, a dominant hot topic during this workshop was the problem of Channel Estimation.