In these notes we describe in some detail a certain framework for doing homotopy theory. This approach emerged in the early 1990's but has roots in earlier work of Bousfield about localization and in the big advances made by Mahowald, Plavenel, Devinatz, Hopkins and Smith towards deeper understanding of the role of periodicity in stable homotopy theory. It is natural to look for a similar unstable organization principle. This has not been found. Rather, certain tools have developed that have proved interesting. In addition, these tools are closely related to the above developments, as well as to central developments that occurred in unstable homotopy with the proof by Miller of the Sullivan conjecture and with the fruitful use of Miller's theorem by Lannes, Dwyer, Zabrodsky and many others.