This paper reviews the technique of apertureless near-field scanning optical microscopy (ANSOM) and its use in mapping the inhomogeneous ferroelectric polarization in BaxSr1-xTiO3 thin films. A preliminary survey compares ANSOM with fiber-based near-field microscopy, highlighting the advantages and limitations of both methods. Interferometric ANSOM is described in detail, including a practical description of how ANSOM images are acquired. A discussion of the various contrast mechanisms in ANSOM is followed by a prescription for eliminating a certain class of topographic artifacts. For the imaging of polarization in ferroelectric thin films, the linear electro-optic effect provides the central contrast mechanism. High-resolution ANSOM images show the existence of polar nanodomains in BaxSr1-xTiO3 films, providing strong direct evidence of its relaxor character.