The biological, technical and computational aspects of this project collectively focused on using electron tomography (ET) for the high-resolution (10-20 nm) 3D reconstruction of entire insulin-secreting beta cells within islets of Langerhans isolated from mouse pancreata. Islets were cultured overnight to represent either steady-state (non-stimulated) or elevated glucose (stimulated) conditions, prior to fast-freezing, freeze-substitution, plastic embedment and cutting into 250-400 nm thick sections for tomographic imaging using intermediate voltage electron microscopy (EM). 3D images (tomograms) of each section were used to evaluate the performance of the new technical and computational approaches developed, and make biological comparisons of intercellular structure-function. Analysis focused on key compartments/organelles of the insulin-secretory pathway - Golgi apparatus, mitochondria, insulin secretory granules and multi-granular bodies. To allow the application of ET to entire mammalian cells, several technical limitations were addressed. Since segmenting (delimiting compartments of interest) tomograms manually, represented the major ërate-limiting stepí of ET, an interactive approach for 3D segmentation using novel interpolation algorithms (crude smooth, pointwise smooth and spherical interpolation) to iteratively predict the shape of 3D surfaces between user-drawn contours was developed. The performance of these tools in segmenting a range of compartment types was examined, and found to significantly enhance the speed and accuracy of manual segmentation. To better compensate for the physical collapse of plastic sections in the EM, a novel method was developed for estimating section collapse by analyzing approximately spherical organelles. Using this method on mature insulin granules in high-resolution datasets, coupled with measurements from the whole cell reconstructions, section collapse was found to be substantially less (~25%) than the value (40%) previously used to re-scale 3D models. Other new approaches developed to further improve the accuracy and quality of tomograms, included interactive tools for fiducial tracking, and the use of larger gold particles, a ëreduced second axisí to account for the missing wedge problem, and deformation grids to account for anisotropic deformation. As well as affording more efficient and precise mapping of cell ultrastructure in 3D for subsequent quantitative analysis, these developments provided new insights for future automated (hybrid) segmentation pipelines and new computational approaches for improving quality and isotropic accuracy of volumetric image data. The Interpolator and DrawingTools for segmentation, AnalysisTools for estimating section collapse and BeadHelper for tracking fiducial particles, written as plug-ins for the IMOD software package distributed by the University of Colorado, are now being used by the wider ET community with significant positive feedback. Using the novel approaches developed, four insulin-secreting beta cells - two from the periphery of an islet frozen 1 hr after stimulation with 11 mM glucose, and two from the periphery of another islet under steady-state 5.6 mM glucose conditions - were reconstructed in their entirety in 3D. Quantitative data on the key compartments/organelles provided new information regarding global changes in cellular organization, and enabled robust comparisons of each pair of functionally equivalent cells at unprecedented spatial resolution. Relative differences in the number, dimensions, architecture and distribution of organelles per cubic micron of cellular volume (including mitochondrial branching) reflected differences in the cellsí individual capacity/readiness to respond to secretagogue stimulation. In the two stimulated cells this was reflected by inverse relationships between the number/size of mature granules versus immature granules, the number/size of mitochondria, and the volume of the trans-Golgi network relative to the entire Golgi ribbon. Complementary stereological analysis of whole islets indicated which cells were the most representative under stimulated versus non-stimulated conditions, and revealed a marked natural heterogeneity between cells both within and between individual islets. Overall, this project led to significant improvements in efficiency and accuracy for segmenting cellular compartments/organelles, and in image quality and accuracy for tomogram computation and reconstruction through use of the newly developed techniques. The improved 3D reconstruction and analysis of pancreatic beta cells in toto in native tissue provided a powerful approach for quantitatively mapping the organelles involved in insulin synthesis/secretion at unprecedented detail, and afforded a level of insight into the complex 3D organization of mammalian cells not previously achieved by any other analytical technique or imaging method.