Nanoscience is one of the most promising research areas in modern science with implausible applications in physics, chemistry, biology and materials science. Nanotechnology can be defined as the design, synthesis and application of materials and devices the size and shape of which have been developed at the nanoscale. In the first chapter, the authors bring to light various green methods used for gold nanoparticles (AuNPs) synthesis, their characterization and highlight their various applications. The second chapter focuses on the facile, low-cost and eco-friendly synthesis of gold nanoparticles using Caesalpinia Crista seed extract and evaluates their antimicrobial, antioxidant and anticancer efficacies. The third chapter focuses on the efficacy of biogenically synthesized AuNPs in the reduction of dyestuff pollutants and in the catalytic reduction of nitrophenols to aminophenols in the presence of NaBH4, as is the usage of plant extract-mediated AuNPs as effective probes for the detection of heavy metal ions in water bodies. The last chapter examines the synthesis, characterizations and applications of gold nanoparticles using a 220 keV Ag Beam.