The study of structure–function relationships of membrane proteins (MPs) has been one of the major goals in the field of structural biology. Many Noble Prizes regarding remarkable accomplishments in MP structure determination and biochemistry have been awarded over the last few decades. Mutations or improper folding of these proteins are associated with numerous serious illnesses. Therefore, as important drug targets, the study of their primary sequence and three-dimensional fold, combined with cell-based assays, provides vital information about their structure–function relationships. Today, this information is vital to drug discovery and medicine. In the last two decades, many have been the technical advances and breakthroughs in the field of MP structural biology that have contributed to an exponential growth in the number of unique MP structures in the Protein Data Bank. Nevertheless, given the medical importance and many unanswered questions, it will never be an excess of MP structures, regardless of the method used. Owing to the extension of the field, in this brief review, we will only focus on structure–function relationships of the three most significant pharmaceutical classes: G protein-coupled receptors, ion channels and transporters.