In this chapter I draw on philosophical literature on (scientific) explanation to assess the goodness of engineering design methods. I focus this analysis on the engineering design practice of reverse engineering and redesign, and elaborate a constraint drawn from the mechanistic explanation literature to assess the goodness of reverse engineering practices and the content of design representations resulting from those practices. This constraint concerns the distinction between causal and constitutive relevance in mechanisms. I spell out two ways in which constitutive relevance assessments give traction to designing: reverse engineering explanation, and design optimization. I end by showing how this analysis fits within and extends recent philosophical work on the interplay between engineering design and explanation, indicating the (broader) relevance and promise of connecting philosophy of explanation and philosophy of design.