The discovery and clinical application of antibiotics is one of the amazing human success stories of the 20th century. Many of the terrifying bacterial diseases have been kept in check and hundreds of millions of lives saved. However, our heavy use of these amazing molecules has come at the cost of selecting resistant strains so that treatment of infections can fail with the first and second line antibiotics chosen. This lecture will introduce antibiotic action and how the adaptability and promiscuity of many types of pathogenic bacteria enables them to develop resistance.