Cryo-electron microscopy allows high resolution structural studies of macromolecules or macro-molecular complexes. As structural analyses extend to higher resolutions, several major compu-tational problems arise in analyzing cryo-electron micrographs. One is the acute sensitivity of the sample to radiation damage, requiring that images be acquired at a low electron dose with conse-quently low signal-to-noise ratio (SNR), especially at higher spatial frequencies. Secondly, as the size of each digitized image grows, the duration of the computational procedures lengthen consider-ably. A third problem is the complex distortion imposed upon the images by the contrast transfer function (CTF) of the electron microscope. We have addressed these issues in the context of solving the Hepatitis B Virus (HBV) capsid structure, and have succeeded in improving the resolution of our model from 17À[4] to 9À[1] (Fig.l). This is sufficient to define part of the molecular structure, including a 4-helix bundle at the dimer interface which constitutes the protruding ‘spike’ domains seen on the surface of the capsid, as well as other helices elsewhere in the molecule.