necessary, needed, important, crucial, vital

necessary, needed, required

to be necessary, to be needed, to be required

obligatory, necessary; definitely, surely

compulsory, necessary, crucial, critical, certain, definite, sure

to be required, needed

to be requested, demanded, called for, needed

necessary, required, obligatory; intransitive (verb - grammar)

required (as of a payment or penalty), at once, immediately, forthwith