The Structural Health Monitoring (SHM) of structures is acquiring a key role in the present time. An in-situ system able to assess the health state of bolted joints would save money and maintenance time, by allowing quick assessment of residual life and degradation state of structures. In this work, linear and nonlinear acoustic/ultrasound techniques were employed to develop a reliable index able to assess the loosening /tightening health state of a bolted structure. The developed indexes were capable of describing the joint behavior at different fastening load. In particular, tightening/loosening state indexes were very well reproduced by an analytical expression where the joint state is expressed as function of the torque applied. The analytical trend approximates the experimental results with excellent correlation.