In this paper, we propose an approach for understanding mathematical expressions in printed document. The overall approach is divided into three main steps: (i) detection of mathematical expressions in a document, (ii) recognition of the symbols present in the expression and (iii) arrangement of the recognized symbols. The detection of mathematical expressions is done through recognition of a few most common symbols and exploiting some structural features of the expressions. A hybrid of feature based and a template-based technique is used for the recognition of symbols. A two-pass approach is used for arrangement of the symbols. The first pass (Scanning or Lexical Analysis) performs a micro-level examination of the symbols in order to identify the symbol groups occurring in them and to determine their categories or descriptors. The second pass (Parsing or Syntax Analysis) processes the descriptors synthesized in the first pass, to determine the syntactic structure of the expression. A set of predefined rules guides the activities in both the passes. Experiments conducted using this approach on a large number of documents show high accuracy.