Understanding virus structure is crucial in order to decipher important features of the viral lifecycle as well as to open up new avenues to interfere with virus assembly.We have been developing cryo-electron tomography and subtomogram averaging tools using retroviruses as model systems, and recently we have obtained high resolution structures from within pleomorphic virus particles (see Schur FKM et al, 2016, Science, 353). This methodological improvement offers a unique potential towards cellular structural biology. We will continue our efforts to reveal the structure of pleomorphic viruses at atomic detail by pursuing our efforts in improving the versatility of the methods, both at the data acquisition and the image processing side. The viruses we are studying include members of the retroviral family, but also DNA-viruses such as Vaccinia viruses or Baculoviruses. For retroviruses we are interested in general features of virus assembly and maturation, a process in which the virus gains infectivity. Vaccinia and Baculoviruses are important model organisms to understand actin-mediated pathogen propulsion.