A nanoparticle can be defined as a small object that behaves as a whole unit in terms of its transport and properties. Nanoparticles are sized between 1 and 100 nm in diameter. Nanoparticles can act against the microbes in multiple ways, and the microbes are less likely to develop resistance against nanoparticles because it requires multiple gene mutations. The large surface-to-volume ratio of nanoparticles, their ability to easily interact with other particles, and several other features make them attractive tools in various fields. Nanoparticles are widely used various fields such as electronics, cosmetics, biomedical, and biotechnology. Nanoparticles can be synthesized by physical methods such as attrition, pyrolysis, and using some wet chemical methods. The physical and chemical methods have various drawbacks such as high cost of production, require high energy input and generation of toxic by-products. To overcome this, several biological methods are employed in the synthesis of nanoparticles. The biological methods are generally cost effective, nontoxic, and ecofriendly. This chapter focuses on the methods involved in algal-synthesized nanoparticles and its applications.