Critical issues remain to be addressed concerning the reliability of in-situ ultrasonic sensors in the structural health monitoring (SHM) domain, where ultrasonic guided waves have been proposed for the frequent monitoring of aircraft joints for fatigue cracks around fastener sites.  In particular, the interface conditions between the fastener shaft and hole are found to vary with time, and often independently of the damage state of the joint.  Changes in the magnitude of scattered ultrasonic signals from fastener sites can result from dynamic structural loading, thermal cycling, aging of sealant present in some structures, and scheduled maintenance actions.  Under certain conditions, the scattered signals from a fastener location can be difficult to distinguish from fatigue crack signals.  Prior work from ultrasonic measurements of fastener sites in aging aircraft has highlighted the variation in fastener-hole fit conditions and the challenge of separating crack signals around the fastener site from re-radiated signals from the fastener hole. To better understand the scattering of ultrasonic guided waves from fastener sites in aircraft structures, analytical models are presented and used for studying the interaction of ultrasonic waves with a cylindrical hole and an elastic insert which are coupled by a stiffness interface.  Parametric studies are presented which investigate the effect of variations in the fastener contact conditions on the reflection of incident Lamb waves and the generation of secondary waves around the fastener hole.