This thesis aimed to investigate the relationship with key customers in the example of the company eIstra.info. At the company itself, it can be seen that the needs and wishes of the clients come first. For easier monitoring of these same wishes and needs, as well as various other things in the company, a CRM system should be modeled. Each CRM system can be modeled according to the company and its needs. It is easier for a company to please and keep an old client than to attract and keep a new one. A loyal client can contribute a lot to a company. The client should be given attention to make him feel special and important. The very communication with the client and how it is communicated with the client has a positive outcome for further cooperation. With the help of the CRM system, I can easily monitor the previous communication with the client. A sign of attention for the client means a lot, tracking birthdays or some special anniversaries, and dates using CRM also shows care for the client and increases his satisfaction. It's true that not all CRM systems are cheap, but over a year, some can pay off even less. Which shows the effectiveness of the CRM system itself. Adapting to entering all data into the CRM system can scare employees because it requires a lot of work, but over time, employees realize how useful the CRM system is and how much it makes their work easier. The CRM system can be used by large and small companies, although Excel is not a bad choice for small companies either. Every company that wants to progress and have satisfied customers will have a CRM system.