This document discusses reference and sense as two aspects of semantics. Reference deals with the relationship between language and the real world, referring to things that exist. Sense deals with relationships within language itself and does not refer to anything in the real world. While an expression can have only one reference, it can have multiple senses. The sense of an expression is its meaning and place within the semantic system of a language. Reference and sense are related but different - an expression has meaning (sense) but not necessarily a reference, and two expressions can have the same referent but different senses.