In its early days, illegal graffiti and street art were of necessity modest in scale, hastily created in hours or even minutes, and destroyed just as quickly by authorities, vandals or the elements. Now, however, graffiti writers and street artists are able to take their work to an entirely new level, creating carefully planned, permanent works that cover entire buildings and add a new visual dimension to the world's cities. Claudia Walde, better known as the record-breaking street muralist MadC, has selected more than 200 of the best XXL works and profiled nearly 30 artists who pioneered this trend.