While there are hundreds of payment processors in the US most of them are actually ISOs (Independent Sales Organizations) or banks.  Developing and maintaining a processing network is expensive and it is easy  and much cheaper for banks and ISOs to just buy access onto one of the existing processing networks.

There are only a few payment processing networks in the US, including:

• RBS Worldpay
• TSYS / Vital
• Elavon, formerly called Nova, owned by USBank
• Heartland
• First Data North
• First Data Nashville (also called Envoy)
• First Data Omaha
• First Data Buypass
• First Data EFSNet
• Paymentech Tampa (also called Gensar)
• Paymentech Salem
• Global Payments East
• Global Payments Central

Payment gateways, POS systems and terminals are usually compatible with one or more payment networks.