In the present work, a single-fiber pullout test was used to study the interface/interphase between various fibers and low-density polyethylene (LDPE) and between glass fibers and a range of other thermoplastic matrices. For well-bonded fibers, experimental evidence suggests the involvement of plastic deformation and strain-hardening prior to debonding and pullout. The interfacial shear strength was determined to be the ultimate shear strength of the matrix and was found to be insensitive to the fiber surface structure. A new theoretical model was developed to predict the relationship between the debonding force and the embedded length. The contribution of friction to the debonding force was found to be insignificant when compared with the contribution of plastic deformation.