The Axial Anomaly and Nucleon Structure

In the Standard Model of particle physics, the axial current is not conserved, due both to fermion masses and to the axial anomaly. The axial anomaly played an important role in the early discussions of the nucleon spin crisis starting in the late 1980s. Several recent studies triggered renewed interest in the axial anomaly and its role in the (spin) structure of the nucleon. We give an overview of the field, discussing recent progress and open questions.