Seaweeds contain compounds from different chemical families, including polysaccharides, fatty acids, phenolics, pigments, lectins, alkaloids, terpenoids and halogenated compounds with reported antimicrobial action. This chapter presents an update of the recent contributions to the evaluation of antimicrobial activity of macroalgae, comprising a brief introduction to the detection methods used for determining antibacterial activity and describes the activity against different microorganisms and viruses. Seaweeds are a great source of bioactive compounds due to the large number of secondary metabolites they synthesize and possess antioxidant and some phytochemical activities. Several authors have determined the antibacterial activity of algal extracts using both in vitro methods, the diffusion agar test and broth dilution method. Assays performed to determine growth inhibition by seaweed extracts were reported by different authors and, in some cases, the use of an indicator of live/dead cells was necessary. For in-depth study on the antimicrobial activity of seaweeds components, several dilution methods as broth dilution tests are used.