Nanotechnology is among the rapidly growing areas of science and technology with the increasing progress being made in the subjects of medicine, cosmetics engineering and electronics. Metallic nanoparticles have extensive medical, consumer and industrial applications due to their unique characteristics such as high surface-to- volume ratio, broad optical and electronic properties, ease of synthesis, facile surface chemistry and functionalization. Exposure of these particles to humans and other biological systems has aroused global concerns regarding their fate in biological systems resulting in a demand for their toxicity assessment. This thesis comprises of two parts. Part I consists of in vitro cytotoxicity, cell viability, mutagenicity and genotoxicity assessment of iron oxide, aluminium oxide and copper nanoparticles and Part II includes in vivo acute oral (LD50 ) toxicity evaluation (iron oxide, aluminium oxide and copper nanoparticles), in vivo genotoxicity and cytotoxicity assessment of iron oxide, aluminium oxide, copper, titanium oxide and silver nanoparticles. In vivo mutagenicity of titanium oxide and silver nanoparticles was also evaluated. Titanium level was determined in mice bone marrow treated with titanium oxide nanoparticles. Mice were exposed intraperitoneally to iron oxide, aluminium oxide and copper nanoparticles for 14 days to evaluate their blood biochemical parameters and histology. In vitro mutagenicity, cytotoxicity, cell viability and genotoxicity of iron oxide, aluminium oxide and copper nanoparticles were evaluated using tests i.e. Ames test, in vitro cytotoxicity assay, cell viability assay, micronucleus assay and comet assay. For mutagenicity assessment, two bacterial strains of S. typhimurium TA98 and TA100 were used. The cytotoxicity to bacterial cells was assessed by using Escherichia coli and Bacillus subtilis in terms of colony forming unit and optical density. In vitro cell viability and genotoxicity of these nanoparticles were determined using the trypan blue assay, comet assay and micronucleus assay following exposure to monkey kidney cell line (CHS-20). In vivo genotoxic potential of iron oxide, aluminium oxide, copper, titanium oxide and silver nanoparticles was observed in mice bone marrow cells using micronucleus assay and comet assay. Furthermore percentage of reticulocytes in the bone marrow of experimental mice was also determined for the evaluation of in vivo cytotoxicity of these nanoparticles. In vivo mutagenicity of titanium dioxide and silver nanoparticles was observed using Pig-a assay. Inductively coupled plasma-mass spectrometry was used to determine the amount of titanium oxide nanoparticles that reached the bone marrow. Serum biochemical analysis was carried out using an autoanalyzer and histological changes were observed by using standard hematoxylin and eosin staining method. In vitro toxicological evaluation showed non mutagenic, non cytotoxic and non genotoxic effects of iron oxide and aluminium oxide nanoparticles. Moreover, no decrease in cell viability was observed for these nanoparticles. Copper nanoparticles showed decrease in cell viability, cytotoxicity, mutagenicity in concentration dependent manner and genotoxicity at the highest tested concentrations. The iron oxide and aluminium oxide nanoparticles were found slightly toxic (oral LD50 more than 2000 mg/kg) and copper nanoparticles were found moderately toxic (oral LD50 value 325 mg/kg). In vivo toxicological assessment demonstrated that iron oxide, aluminium oxide, titanium oxide and silver nanoparticles were not genotoxic and cytotoxic. However, copper and titanium oxide nanoparticles were found cytotoxic (decrease in percentage of reticulocytes). Only copper nanoparticles showed a significant increase in micronuclei and DNA damage at a highest tested dose. The results from inductively coupled plasma-mass spectrometry suggested that the titanium oxide nanoparticles reached the bone marrow, the target tissue for the genotoxicity assays. No changes in serum biochemical parameters were observed in mice treated with iron oxide and aluminium oxide nanoparticles. Serum elevated level of liver enzymes was observed in mice treated with copper nanoparticles along with mild to moderate vacuolation in hepatocytes. The finding of this thesis will advance the knowledge about the toxicological effects and safety of metallic nanoparticles in view of their tremendous applications in various fields of life.