This report presents the results of an investigation of the fatigue strength of structural lugs. The program included both experimental and analytical phases which were used in a complementary fashion to formulate design charts for fatigue-loaded steel and titanium lugs containing interference-fit liners. These lugs are representative of design practice in highly loaded aircraft applications, particularly that found in helicopter blade attaching systems. A primary element in the analytical study was a two-dimensional structural analysis of lug configurations, which was done by finite element methods using a computer program. This computer program is published in Volume II of this report. The design charts presented will permit the designer to rapidly select lug proportions in either steel or titanium that will satisfy structural requirements for a range of steady and vibratory loading. The designs are considered to be particularly applicable to helicopter rotor and control systems.