In this book, a summary and update of the most important areas of CPP research are presented, whilst  raising relevant questions for further development.  The CPP sequences are presented and discussed throughout the book. The methods for testing CPP mechanisms are discussed in detail. Various approaches for the testing of endocytotic pathways of CPP uptake are also described. Different CPP uptake experiments are compared since it is becoming clear that it is often best to apply several methods in a complementary manner in order to most comprehensively evaluate CPP uptake mechanisms due to the complexity of these processes. A brief summary of functionality issues of CPPs, both in vitro and in vivo are discussed. Therapeutic potential of CPPs and commercial developments are discussed.  The monograph is written for researchers and students in the field.