Pre-judgment interest, or interest before there is a lawsuit and court judgment against the debtor, can only be charged if there was something in the paperwork or agreement enabling the assessment of interest. (Though note: it may be enough if the invoice or bill stated that if not paid, there would be interest.) Basically, prior to a court judgment, what can be charged is what the sales, service, etc. agreement says can be charged, including potentially interest, late fees, and collection or legal osts.Post-judgment, or after a winning lawsuit, the laws allow for the winning plaintiff to recover interest (called post-judgment interest) if the damages awarded in the judgment are not paid in a timely fashion. So if there's been a judgment already, a creditor or its collections agency can recover interest even if there nothing to that effect in the agreement.