Because of their wide scope of application within the firm, ERP software systems rely on some of the largest bodies of software ever written. Implementing such a complex and huge software system in a company used to involve an army of analysts, programmers, and users, until the development of the Internet allowed the use of outside consultants to connect to company computers to install standard updates. ERP implementation, without professional help, can comprise a very expensive project in itself for bigger companies, especially transnational, but with companies specializing in ERP implementation, the task can be completed in under six months, with solid pilot testing. 

Enterprise resource planning systems are often closely tied to supply chain management and logistics automation systems. Supply chain management software can extend the ERP system to include links with suppliers. 

To implement ERP systems, companies often seek the help of an ERP vendor or of third-party consulting companies. Consulting in ERP involves two levels, namely business consulting and technical consulting. A business consultant studies an organization's current business processes and matches them to the corresponding processes in the ERP system, thus 'configuring' the ERP system to the organization's needs. Technical consulting often involves programming. Most ERP vendors allow modification of their software to suit the business needs of their customer. 

Customizing an ERP package can be very expensive and complicated, because many ERP packages are not designed to support customization, so most businesses implement the best practices embedded in the acquired ERP system. Some ERP packages are very generic in their reports and inquiries, such that customization is expected in every implementation. It is important to recognize that for these packages, it makes more sense to buy third party reporting packages that interface well to particular ERP, than to reinvent what tens of thousands of other clients of that same ERP have needed to develop.

Source: http://www.cio.com/research/erp/edit/erpbasics.html