During works on the finished product, that is used by users, A / B testing is a useful tool. It is a research method based on comparing two versions of a website to select one that is better in meeting the functional requirements. It is used to increase conversion. Unlike usability testing on prototypes, A / B testing allows you to test new solutions by making driven data decisions. Very often seemingly small differences, such as changing the color or size of the button, will enable you to increase the conversion and significantly increase profits. A / B testing is based on presenting some users entering the website its alternative version. Specialistic tools like VWO or Optimizely track these users sessions. It allows you to generate relevant reports and heat maps, used to visualize the users' traffic on the website. All this is aimed at selecting the best possible solutions based on real data.