This document provides an overview of telecom billing solutions. It discusses the evolution of billing systems from in-house developed systems in the 1980s to more complex third-party solutions today. The general billing process is described involving collecting call detail records from the network, rating calls according to customer plans, aggregating rated calls into invoices, and generating bills. The key components of a billing system including mediation, rating, and invoicing engines are explained. Finally, the document outlines the major functions performed by rating engines in processing usage records and preparing them for invoicing.