Semantica is the manual for a new software application that allows the user to explore the semantic structure of language in an engaging, interactive way. The program, which was produced as part of a National Science Foundation initiative for improving linguistics instruction, is designed to be used with Syntactica, a tool for studying natural language syntax. Semantica provides a simple graphical interface for creating semantic theories, viewing the truth conditions that those theories assign to phrase-markers created in Syntactica, and for testing those truth conditions in a pictorially represented world. Although designed for use in introductory semantics courses, Semantica has features that will appeal to professionals. For example, it can be used as a "derivation calculator" in following out complex semantic derivations involving many steps and operations. Manual plus NeXTStep software for PCs and NeXTStationsrunning NeXTStep version 3.2 or higher.