Antiphase boundaries (APBs) are rather common extended defects appearing in crystals with ordered sublattices. They are formed when two parts of the crystal are specifically shifted one with respect to the other. Here we report on a series of quantum-mechanical studies focused on impact of antiphase boundaries on thermodynamic, elastic, structural, magnetic and vibrational properties of magnetic materials. We show, for example, that antiphase boundaries can increase the total magnetic moment in disordered Fe-Al alloys (Nanomaterials 10 (2020) 44) or partly-ordered Fe-Al-Ti intermetallics (Crystals 9 (2019) 299) and can be employed when designing new magnetic materials. When comparing phonons in the Fe3Al with and without APBs, we find a clear influence of APBs that can, e.g., significantly reduce the gap in vibrational frequencies (Materials 13 (2020) 4884). We also demonstrate that properties of APBs very sensitively depend on the chemical composition of their interface (Magnetochemistry 7 (2021) 137) and can be fine-tuned by segregating elements such as Cr (Materials 12 (2019) 3954) or Cu (Magnetochemistry 7 (2021) 108) in Fe3Al. The presentation summarizes the impact of APBs on properties of selected magnetic alloys and intermetallic compounds and focuses on fundamental features of APBs and their behavior.