Shaping of the root canal is one of the most important stages of root canal treatment, which determines the ability to thoroughly disinfect and fill the root canal system. Numerous instruments have been designed for this purpose. Over the past twenty years, rotary nickel-titanium endodontic instruments have become the most prominent innovation in this field. This group of instruments possesses well-known advantages, compared with traditional stainless steel files. There are also some weaknesses, several of which could be explained by alloy characteristics and fabrication method, and others due to design characteristics. In the available literature, detailed information concerning design parameters and their influence on the instrument behaviour in the root canal is scarce. As a rule, a manufacturer's instructions are limited to some empirical recommendations for users. Meanwhile, understanding the instrument's properties and differences in its basic design features is essential for determining its operative abilities and correction of its disadvantages, by means of improving the technique or by taking into account its limitations, to avoid undesirable complications. Schlagwörter: angles of cutting blade, cross section, design features, flute design, rotary nickel-titanium instruments, self-feeding effect, taper