An X-ray
FEL project, such as the Linac Coherent Light Source (LCLS), calls for
the production of high energy bunches of electrons that are simultaneously
short, intense, and have small emittances. Wakefields of the accelerator
structures in the linac of the LCLS are an important ingredient in longitudinal
phase space manipulation; other wakefields can degrade the beam emittance
and, thus, the FEL performance. In this talk we discuss wakefields of
extremely short bunches, as we find in the LCLS. In this regime of bunch
length relatively new types of wakefields become important (e.g. roughness
wake), and familiar wakefields exhibit new types of behavior (e.g. resistive-wall
wake). In conclusion, we assess the importance of different wakes to
LCLS performance.