The emergence and spread of multidrug resistance among human pathogenic bacteria is an increasing worldwide problem. A comprehensive understanding of the molecular basis of resistance mechanisms of bacteria will be vital for the future development of new and more effective antibiotics and for novel therapeutic treatment strategies. One of the common resistance mechanisms is the active efflux of toxic compounds from the cell by bacterial multidrug efflux systems, which are polyspecific and able to accommodate a variety of structurally and functionally unrelated compounds. Moreover, it is well recognized that the expression of these multidrug efflux transporters is tightly controlled by transcriptional regulators with the same multidrug recognition properties. Elucidation of structural and functional relationships of these multidrug efflux transporters and transcriptional regulators is the major concern of this dissertation. In order to understand the role of Rv3066 in regulating the expression level of the multidrug efflux pump Mmr in M. tuberculosis, we have determined the crystal structure of Rv3066, both in the absence and presence of ethidium bromide. With the aid of other experimental data, induction mechanism of Rv3066 is studied in detail. Currently available various methodologies to crystallize membrane proteins have also been outlined in this dissertation. Towards understanding the molecular mechanism of MtrCDE efflux pump of N. gonorrhoeae, which mediates the export of several structurally diverse toxic chemicals, we have determined the crystal structure of MtrD. In addition, the Mtr efflux system includes another inner membrane protein MtrF, which belongs to the AbgT family of transporters. To date, approximately 13,000 putative transporters of this family have been identified. However, no structural information has been made available and even functional data are fairly minimal. To understand how members of the AbgT family function, we have determined the crystal structure of MtrF and combination of functional studies suggests that MtrF acts as an antibiotic efflux pump. In addition, we have exploited our knowledge on AbgT transporter family, by determining the crystal structure of another member YdaH, of this family. Based on our observations, we believe that many members of the AbgT family transporters may serve as antimetabolite efflux pumps to protect cells against these noxious agents.