Penicillins, a class of β-lactam antibiotics initially derived from Penicillium moulds, revolutionised antimicrobial therapy following their discovery by Alexander Fleming in 1928. Characterised by a unique β-lactam ring structure, penicillins disrupt bacterial cell wall synthesis, primarily targeting Gram-positive organisms. Semi-synthetic derivatives, including aminopenicillins and ureidopenicillins, broaden their efficacy to encompass certain Gram-negative pathogens. Clinical applications range from treating uncomplicated urinary tract infections to life-threatening conditions such as sepsis. Resistance mechanisms, including β-lactamase production and structural bacterial adaptations, have necessitated innovations like β-lactamase inhibitors. Penicillins remain pivotal in both community and hospital settings, with careful consideration for prior reported allergy, resistance patterns, and pharmacokinetics to ensure effective and safe use.