  We revisit some aspects of D-brane theory from the point of view of
anomalies. When the boundary condition on a worldsheet boson is flipped from
Neumann to Dirichlet, worldsheet supersymmetry requires also reversing the sign
of the boundary condition of the corresponding worldsheet fermion. This induces
an anomaly which is a mod 2 anomaly in Type II superstring theory and a mod 8
anomaly in Type I superstring theory. The same anomaly also receives
contributions from a sign in the sum over bulk spin structures (in Type IIA
superstring theory), Chan-Paton factors of symplectic type (in Type I
superstring theory), and Majorana fermions that propagate only on the
worldsheet boundary. The need to cancel the anomaly accounts for many
properties of supersymmetric and especially nonsupersymmetric D-branes in Type
I and Type II superstring theory.
