Aluminum (Al) is the most common metal in the earth's crust and, with its natural occurrence and use in consumer products it is ubiquitous in human life. Al compounds are used in food packaging for food protection, but also as stabilizers, pH regulators and anti-caking agents. However, adverse health effects of Al have been discussed and neurodegenerative diseases have been linked to Al exposure. In the production and application of Al, nanoscaled particles are used intentionally but also unintentionally. Due to the unique characteristics of nanoparticles (NP), these can exhibit a different behavior than the respective bulk material. Their altered characteristics may also affect for example the oral uptake. A higher bioavailability due to increased solubility would also be conceivable. This may lead to a faster exceedance of the limit values for Al uptake. So far, the tolerable weekly intake value for Al from food do not differentiate between different forms of Al, such as ionic or nanoscale species. To gain a better understanding on whether Al NP could potentially cause adverse health effects, two nanoscale Al particles of similar size were characterized in this thesis by in vitro and in vivo experimentation. The obtained results were also compared to an ionic Al control. For any meaningful in vitro or in vivo investigation, a proper and comprehensive characterization of the applied NPs is mandatory. This requires appropriate analytical techniques and the knowledge about their advantages and limits. The first part of this thesis compared different analytical methods for NP characterization. Particular attention was paid to techniques that can determine solubility and complexation behavior, since NPs can exhibit different properties in physiological media. To evaluate the possible differences between nanoparticulate metal and metal oxide Al forms, two similar sized particles, Al and Al2O3 NPs, were chosen. Fast screening techniques were evaluated for their suitability of characterizing hydrodynamic diameter and size distribution. More specialized techniques were applied to determine individual NP numbers and size. The use of complementary techniques confirmed the obtained results and highlighted the benefit of using more than one technique for a comprehensive NP characterization. With particle surface, another important property of NPs was investigated, revealing an important difference between the metal and metal oxide Al NPs used. For all techniques applied, advantages and disadvantages were discussed, and possible complementary techniques suggested. The results obtained served as a basis for the following in vitro and in vivo studies. During oral uptake, NPs from food or food contact material may reach the gastrointestinal tract. The second part of this thesis assessed the physicochemical fate of Al and Al2O3 NPs after passing through different compartments of the gastrointestinal tract. For this purpose, an artificial digestion approach was applied. A broad spectrum of techniques was used to characterize particle modifications during their passage. For comparison, ionic Al was used as a control. Three compartments, saliva, stomach and intestine, of the gastrointestinal tract were monitored. For saliva, no significant differences in particle size and shape were identified. In the gastric fluid, a strong agglomeration and an increased ion release were detected. In contrast, the subsequent exposure step to the intestinal fluid led to a reverse behavior, resulting in deagglomeration and decrease of free Al ions for all three Al species. A de novo formation of Al protein complexes in the nanoscale range was shown in intestinal fluid, even for the ionic Al control. The results suggest the possibility of nanoparticulate fractions reaching the intestinal epithelium. Different modifications and transitions may alter these NPs significantly. These findings stress the importance and necessity of a thorough characterisation of the NP under real-life conditions. It is suggested to consider a mutual conversion of particles and ions when evaluating toxicological experiments. The oral uptake is a complex process and important factors like bioavailability and tissue distribution cannot be monitored in vitro. The third part of this thesis focused on a three-day oral gavage study with three different Al species (Al and Al2O3 NPs and ionic AlCl3) in Sprague Dawley rats. For selected organs, the Al amount was determined to identify the systemic available Al burden and distribution. Three different concentrations of the two NP species were administered, while the ionic Al control was applied at one dose only for comparison. The development of a microwave assisted acid digestion approach followed by inductively coupled plasma mass spectrometry (ICP-MS) analysis enabled the quantification of Al burden of individual organs. Due to the complex matrices, sample matrix interferences on the calibration procedure were considered. After three days of exposure, the three Al species tested showed a different distribution behavior within the investigated organs. High Al levels were found in liver and intestine for the ionic Al control, while upon treatment with Al NPs significant amounts of Al were detected in the latter only. High concentrations were detected in spleen following Al2O3 NP treatment, however in all other investigated organs Al was also determined. After the three-day oral exposure via gavage, for all three Al species a rapid absorption and systemic distribution was observed. The results suggest particle shape and surface composition as key factors for Al biodistribution and accumulation. In summary, this thesis provides a strategy for a comprehensive characterization especially of aluminum-containing NPs, taking into account the advantages and limitations of known techniques for NP analysis. In addition, differences in both physicochemical properties and behavior of the two similar sized NPs used in physiological environments were determined. The results obtained here provide robust evidence that when evaluating Al after oral intake, when establishing recommended tolerable intake values, and when conducting future toxicological studies, a distinction must be made between different forms of Al, such as ionic or nanoparticulate species.