Hardware verification is the art of proving formally that, to within the tolerance of an underlying model, a design meets (or perhaps does not meet) its specification. This paper is an introduction to hardware verification and its limitations. We illustrate the technique by specifying and verifying an x or gate and a ripple carry sub-system using the HOL notation, (see [2,8,9]), and then demonstrate its capabilities with sample applications to VLSI CAD and system re-implementation.