The antonyms for "non-compulsory" are mandatory, obligatory, required, necessary, essential, and imperative. These antonyms suggest that something is required or necessary and must be done or followed. Use of these antonyms implies that a certain action or behavior is expected or enforced, and there is no room for choice or exception. For instance, mandatory has a very strict connotation, implying that something is required by law, regulation, or authority. The word essential, on the other hand, implies that something is crucial or indispensable, and its absence would render a task or activity incomplete or ineffective. Therefore, these antonyms are useful when describing situations that are compulsory or necessary, as opposed to optional or voluntary.