The basic decoding of MERFISH data\u2014the identification of specific RNA species for each spot in the sample\u2014consists of three basic steps. First, fluorescent spots must be identified in all images. Second, slight changes in the stage position between images of the same region from different hybridization rounds must be corrected. Finally, sets of fluorescent spots that occupy the same location in the sample must be decoded into binary barcodes, which, in turn, are decoded into the specific RNA species. After these RNAs have been decoded, there are a series of simple calculations that can often be performed on the measured barcodes to determine important parameters associated with the performance of the measurements, such as estimates of the binary errors made at each bit or the relative confidence that can be ascribed to the counts for each RNA species. In this section, we provide protocols for performing these basic computational tasks. Example software and data can be found at: http:\/\/zhuang.harvard.edu\/merfish\/.