• Controls the flow of logic
• test may be any statement
• If test returns NIL/FALSE then the statements after THEN (if any) are executed
• If test does not return 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-NOT-TRUE... and IF-NOT-SUCCESSFUL... are synonymous with IF-FALSE...

Example:
  (IF-FALSE (FIND-ELEMENT (ORGANISM-OF gene) IN filamentous-organisms)
      THEN (ADD-TO-LIST set1 gene)
      THEN (ADD-TO-LIST set2 gene))

[If the name of the given gene's organism cannot be found in a list of organisms, then the gene is added to one set. Otherwise it is added to another set.]