Decimal numbers are introduced in the fifth grade of primary school, and their application is not only in school but also outside it, so we can easily apply what we have learned about them in the needs of everyday life. In this paper, we explained how decimal numbers should be processed in school. We learned about different models by which we can approximate decimal numbers to students, we pointed out what every teacher should pay attention to, and that decimal numbers should be processed step by step for better understanding. We also pointed out that for a better understanding of decimal numbers, it’s important to understand and comprehend lectures that precede them, ie. natural numbers, divisibility of natural numbers, and fractions. We showed the most common mistakes to students and teachers and what to pay more attention to while processing the curriculum. In addition to all that, there are some tasks/activities that are interesting on the one hand, on the other hand, they help students to understand the concept of a decimal number in a quality, meaningful and understandable way. We have emphasized that assessment and reasoning are important in basic operations with decimal numbers, and only then algorithm to encourage students to develop logical thinking that is crucial in mathematics.