Erythromycin, the first antibacterial macrolide introduced into the clinical setting over 50 years ago, was used extensively not only for the treatment of respiratory tract infections in both adults and children, but also for bone and soft tissue infections, and specific sexually transmitted diseases. Macrolide antibiotics have undergone a dramatic chemical evolution over the past 50 years, culminating in the improved 14- and 16-membered macrolides, acylides and new ketolides. In all cases, improvements in antibacterial activity involved changes in the interplay between the chemical structure of the macrolide and the components of the bacterial cell that dictate ultimate antibacterial activity and efficacy. Target site modification by methylation of ribosomal RNA, the so-called Macrolide-Streptogramin-Lincosamide, (MLS0 resistance and active efflux are the two most common forms of resistance present in the clinic today; however, other resistance mechanisms are known. The first macrolide that bound to MLS-resistant ribosomes was reported in 1989, demonstrating that appropriate structural changes could regain access to the modified ribosome-binding site. In addition, macrolide analogs with reduced affinity for the active efflux pump were identified in 1990, demonstrating that features of pump recognition could be separated from ribosome binding site recognition. Progressive medicinal chemistry led to the synthesis and development of the more recent ketolide class, which combines attributes of both prototypes into one molecule, i.e. non-recognition by the efflux pump and regaining some access to the modified ribosome binding site. Ketolide also lack of induction of erm methylase as do 16-member macrolides. This review will focus on the biological properties of macrolides in terms of their complex interaction with bacterial cells and 50S ribosome target.