Book excerpt: The book contains original contributions from leading international scientists working in the field of multiphoton absorptions and excitations and their practical applications. From the first theoretical prediction of the phenomena at the beginning of the nineteen thirties through the first practical demonstration seen some thirty years later, the topic has today evolved into a large branch of nonlinear optics. The different chapters of the book treat this large research field comprehensively: from basic phenomena and their description, material design, synthesis, elaboration and optimisation, through experimental characterization techniques to practical applications such as optical power limiting, nano and microstructuration and photodynamic therapy. Although the contributions address almost exclusively organic materials, currently the group know to have the largest multiphoton transition cross sections whose properties can be tailored for targeted applications, the described characterization techniques as well as applications are universal and can be done with other materials. The book is addressed not only to the researchers active in the field but also to graduate students, newcomers and development engineers interested in practical application of multiphoton induced effects.