The proton is not "fundamental" in the same way as the point-like electron. In fact, it is a composite system composed of quarks and gluons. When we probe a proton with an electron beam in inclusive scattering, the deviation from simple point-like behavior is characterized by four structure functions, each of which describes a particular aspect of the proton's compositeness. Three of the four structure functions are fairly well determined. The fourth structure function g2^p is relatively unknown and its knowledge is critical for a full understanding of the simplest bound atomic systems, notably, the hydrogen atom, which stimulated the construction of QED. Currently, one of the main uncertainties in understanding these simple bound systems comes from our knowledge of g2^p. We will discuss recent results from the Jlab spin structure program and give a perspective on upcoming experiments.
