The primary objective of this study has been to examine the behavior of continuous composite beams. The investigation has been based on a nonlinear analysis of 15 composite floor systems. New design criteria have been proposed to calculate the effective width at mid-span and supports, at yield and at ultimate loads. New design equations have been developed to calculate the ultimate strength of a composite section in both positive and negative moment regions. Ductility and deflection characteristics have been studied and equations are proposed to calculate the deflection at service load.