This thesis purports the electron and X-ray microanalysis of planetary materials: from Comet 81P/Wild2 to the surface of Mars. Advanced techniques in electron microscopy and X-ray spectroscopy have been developed for the microanalysis of the nakhlite martian meteorites and Comet 81P/Wild2 samples from the Stardust Mission. Electron microprobe analysis and a Focussed Ion Beam - Scanning Electron Microscope (FIB-SEM) technique for Transmission Electron Microscopy (TEM) was used to analyse the secondary mineral assemblages in the nakhlite martian meteorites. Using these techniques, a model is proposed describing the formation of the nakhlites? secondary assemblages by an impact-induced hydrothermal system based on the mineralogical and geochemical differences between different samples. A suite of Stardust cometary samples have also been analysed using FIB-TEM and microfocus X-ray spectroscopy that includes: X-ray Fluorescence Spectroscopy (XRF), X-ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) at the Diamond synchrotron, the understand the composition of Comet 81P/Wild2 and Jupiter Family Comets.