How you bill your clients for your consulting work depends on your consulting contract or, in the absence of a written contract, any other agreements you may have made regarding billing and payment. If your contract is set up to pay you as you achieve specific milestones — say, completing one-quarter of the project or submitting a draft report of recommendations — then you invoice your clients upon completion of each milestone. However, if you work continually and bill your clients for the number of hours that you work plus expenses (photocopying, travel), then you collect the charges and bill them to your client at the end of the agreed-upon billing period, usually the end of each calendar month.