Nanomaterials possess unique and beneficial chemical, physical and mechanical properties and are used in a wide variety of applications, e.g. electronics, consumer products, paints, clothing, footwear, cosmetics and food. The nanoscale size ranges from 1 nm to 100 nm and some properties will be typically exhibited in this size range. To regulate the use of nanomaterials in consumer foods, the European commission has proposed a nanomaterial definition. To implement this definition, transmission electron microscopy is considered a key method for nanomaterial characterization, because of its high resolution.In this dissertation, standard operating procedures for specimen preparation, measurement of nanoparticles and fractal analysis of fractal like nanomaterials by transmission electron microscopy are developed and evaluated. Based on these procedures, automated image analysis approaches are developed and validated on near-monomodal near-spherical nanomaterials. Next, these validated procedures are then applied to characterize the aggregates and primary particles of powdered nanopaterial. Finally, the particle tracking analysis technique is investigated as an alternative for transmission electron microscopy for the measurement of particle size in the framework of the nanomaterial definition. This technique is validated for the measurement of the primary particle size of near-monomodal near-spherical nanomaterials. The transmission electron miscrosopic semi-automatic image analysis of electron microscopy micrographs allows characterizing nanomaterials by measuring 23 measurands that describe the size, shape and surface topology of nanoparticles. To this end, the validated area equivalent circular diameter measurement by transmission electron microscopy replaces the variety of one dimensional size measurements. For this purpose certified silica reference materials with certified values for the area equivalent circular diameter are tested under repeatability and intermediate precision conditions. To cover a broader range from 8 nm to 200 nm the method was further validated on colloidal gold and polystyrene materials.This work shows that particle tracking analysis can replace transmission electron microscopy for the characterization of near-monomodal near-spherical materials in the size range between 30 nm and 200 nm when the particles are in a stable dispersion and the scattered spot can be distinguished from the background. This ability is demonstrated through an intra-laboratory comparison study which measures the primary particle size with both particle tracking analysis and transmission electron microscopy.A suitable and generally applicable sample preparation is developed to bring powdered synthetic amorphous silica, titanium dioxide and carbon black materials in a stable dispersion in water and cell-culture medium. The aggregates that compose these materials can be characterized and the influence of sample preparation can be evaluated. To obtain this stable dispersion a sonication based sample preparation is developed and a standard operating procedure is developed to coat these particles to electron microscopy grids.An approach to replace the manual measurement of primary particles in fractal like titanium dioxide nanomaterials by a semi-automatic approach is developed. To determine the primary particle size, particles are detected using watershed segmentation and their size is measured on the Euclidean distance map. In addition, Standard operating procedures are developed to manually measure the primary particle size and to measure the fractal characteristics of these nanomaterials.This work established that transmission electron microscopy and particle tracking analysis are now validated techniques for the semi-automatic characterization of near-spherical near-monomodal nanomaterials. The next generation validation studies will focus on more complex models. For TEM, a first step in this direction is elaborated in the second part of this dissertation where the validated technique is applied to more complex models, where new strategies are described to disperse powdered nanomaterials and to measure the primary particle size in aggregated nanomaterials.The results of this work are particularly interesting since they allow characterizing both simple and complex colloidal aggregated and agglomerated nanomaterials. These achievements are significant steps forward towards characterization of nanomaterials for risk assessment.