Background/purpose: Investigation into the misconceptions of preschool students in mathematics and their differences between the ages of 4-5 and 5-6 years old helps form appropriate developmental mathematics teaching programs. However, although several studies have been conducted examining preschoolers' previous knowledge and misconceptions about mathematics, no corresponding research has been found in Greece. This study aims to investigate preschoolers' misconceptions about numbers and operations and to reveal differences between preschoolers aged 4-5 and 5-6 years old. Materials/methods: Data were collected through semi-structured interviews and analyzed according to content analysis methodology. Results: The study's results showed that 5-6-year-old preschoolers perform better than those aged 4-5 years old. Most misconceptions of the latter group appeared to be related to reverse counting, identifying arithmetic symbols and their matching quantities, adding and removing numbers without using auxiliary objects and multiplication. On the contrary, some preschoolers aged 5-6 years old needed help adding or subtracting two-digit numbers without the use of auxiliary objects. There were also a few cases where cardinality, division, and multiplication were observed. Conclusion: The misconceptions identified in the two groups of students regarding numbers and operations and their distinct needs that emerged through the research will allow teachers to offer differentiated instruction and personalize teaching support.