Multidrug efflux proteins, also known as efflux pumps, are one of the major mechanisms that bacteria have evolved for their resistance against antimicrobial agents. Gram-negative bacteria are intrinsically more resistant to many antibiotics and biocides due to their cell structure and the activity of multidrug efflux proteins. These transporters actively extrude antibiotics and other xenobiotics from the cytoplasm or surrounding membranes of cells to the external environment. Based on amino acid sequence similarity, substrate specificity and the energy source used to export their substrates, there are seven major families of distinct bacterial multidrug efflux proteins: ABC, RND, MFS, SMR, MATE, PACE, AbgT. Individual proteins may be highly specialized for one compound or highly promiscuous, transporting a broad range of structurally dissimilar substrates. Protein structural organization in a large majority of the families, including the number of transmembrane helices, has been confirmed by high-resolution structure determination for at least one member. In this book chapter, we provide an updated review on the families of bacterial multidrug efflux proteins, including basic properties, energization, structural organization and molecular mechanism. Using representative proteins from each family, we also performed analyses of transmembrane helices, amino acid composition and distribution of charged residues. Ongoing characterization of structure-function relationships and regulation of bacterial multidrug efflux proteins are necessary for contributing new knowledge to assist drug development and strategies that will overcome antimicrobial resistance.