Three-dimensional electron microscopy (3DEM) has made significant contributions to structural biology. To accomplish this feat, many image-processing software packages were developed by various laboratories. The independent development of methods naturally implied the adoption of dissimilar conventions-penalizing users who want to take advantage of the wealth of algorithms from different packages. In addition, a public repository of 3DEM research results, the EM Data Bank, is now established. In an era where information exchange is important, standardizing conventions is a necessity. The 3DEM field requires a consistent set of conventions. We propose a set of common conventions named the "3DEM Image Conventions." They are designed as a standardized approach to image interpretation and presentation. In this regard, the conventions serve as a first step on which to build data-exchange solutions among existing software packages and as a vehicle for homogenous data representation in data archives, such as the EM Data Bank.