The Shack-Hartmann wavefront sensor is a powerful tool for optical analysis. A vital step in the data analysis chain involves reconstructing the wavefront incident upon the lenslet array from hundreds or thousands of slope measurements. We discuss here the basics of reconstruction and the differences in reconstruction for the case of Hartmann and Shack-Hartmann sensors. Hartmann sensors take point samples of the wavefront but Shack-Hartmann sensors measure the average slope over small regions. This results in subtle differences in the reconstruction.