The concept of monochromatic wavefront aberration is discussed in relation to ray aberration, and methods for its measurement in the case of the eye are described. Examples are given of the measured aberration in individual eyes: in general the aberration falls within the Rayleigh quarter-wavelength criterion for pupil diameters less than 2 to 3 mm but significantly degrades image quality for larger pupils. The use of wavefront data to explore both the modulation and phase transfer functions of the eye as a function of pupil diameter and focus is described, together with typical results. Potential applications to clinical problems such as the effects of contact lenses, intraocular implants, refractive surgery, and pathology on the optical quality of the eye are outlined.