• Controls the flow of logic
• test may be any statement
• If test returns anything but NIL/FALSE then the statements after THEN (if any) are executed
• If test returns NIL/FALSE then the statements after ELSE (if any) are executed
• THEN is required but ELSE is optional
• The value of the last statement executed is returned by IF-TRUE
• If no statement is executed, then NIL/FALSE is returned
• IF-SUCCESSFUL... is synonymous with IF-TRUE...

Example:
  (IF-TRUE (> (LENGTH-OF gene) 300)
      THEN (PROTEIN-OF gene)
      ELSE (INCREMENT small-genes)
           NIL)

          [If a given gene has a length greater than 300, then the corresponding protein is returned.
            Otherwise the variable containing the number of small genes is incremented, and NIL is returned]