As computers move onto the desks of all kinds of workers, and into all aspects of the work we do, online documentation becomes increasingly important. Documentation for a computer system or program contains the information that a user needs to know in order to operate the system or program. Online documentation refers to the effort to deliver some or all of that documentation using the same computer system that the user is learning about. This paper surveys many of the issues involved in designing online documentation: choosing a goal for online documentation, understanding the constraints on a design, and understanding the implications of a design for writers and users, mostly from the perspective of a designer or project planner concerned with issues in both software development and document development.