This master’s thesis describes theory of e-commerce, payment card payment model and payment gateway problematic. With the appearance of the internet begins the development of e-commerce which facilitates payment process. Every e-commerce application has implemented some form of payment gateway and uses certain form of electronic payment. The most popular electronic payment model is payment card model. Payment gateway plays important role in a payment card payment model. It acts like a secured bridge for communication between issuing and acquiring banks. Payment gateway must comply with the PCI DSS security standard. In the end of this master’s thesis we will develop a prototype of e-commerce with a built-in payment gateway for PayPal Express Checkout service. Application is developed in cooperation with Adacta.