In this paper we aim at applying established formal methods techniques to a recent software area: quantum programming. In particular, we aim at providing a stepwise derivation of Grover?s quantum search algorithm. Our work shows that, in principle, traditional software engineering techniques such as specification and refinement can be applied to quantum programs. We have chosen Grover?s algorithm as an example because it is one of the two main quantum algorithms. The algorithm can find with high probability an element in an unordered array of length L in just O(pL) steps (while any classical probabilistic algorithm needs (L) steps). The derivation starts from a rigorous probabilistic specification of the search problem, then we stepwise refine that specification via standard refinement laws and quantum laws, until we arrive at a quantum program. The final program will thus be correct by construction.