Substituted barium titanate counts among the best-performing lead-free perovskite relaxor ferroelectric materials. It is suitable for application as actors, sensors, and electrical energy storage systems in microelectronics due to its thermal stability, high permittivity, and low coercivity. One way to induce relaxor ferroelectric (short: relaxor) behaviour is replacing the central B-site Ti[4+] ion with foreign ions like Zr[4+] (homovalent), Nb[5+] (heterovalent), or simultaneous substitution of both species (co-substitution). Homovalent substituents impede a collective displacement of the O¿Ti¿O chains via the introduction of random elastic fields, whereas in heterovalent substitution a perturbation is caused by random electric fields as a consequence of local charge imbalance. Even though relaxors have been investigated for more than half a century, numerous fundamental questions about their unique behaviour among dielectrics remain unanswered. This master's thesis comprehensively investigates the influence of homovalent, heterovalent, and co-substitution on the ferroelectric properties of polycrystalline barium titanate. A suitable tool for this investigation is Piezoresponse Force Microscopy (PFM), a special mode conducted by an Atomic Force Microscope. Single Frequency PFM enables the mapping of the ferroelectric domain structure, which can be used to infer the effectiveness of substituents in perturbing the long-range correlation between the polarization vectors in the material. In addition, local poling experiments, where a bias is applied to the conductive tip of the Atomic Force Microscope, can be conducted to probe the ferroelectric switchability of the material and the long-term stability of newly formed domains. An even more sophisticated method, Switching Spectroscopy PFM, gives insight into the local polarization switching dynamics in the zone underneath the conductive tip, for the active-field and field-free case. Multiple measurements of this type within a defined temperature interval, combined with a semi-automated algorithm for correction, cleaning, and evaluating the generated data, provide information about the temperature-dependent local polarization switching dynamics in ferroelectrics and relaxors. The results reveal that co-substitution is most effective in disrupting ferroelectric long-range order, followed by heterovalent Nb[5+] substitution and Zr[4+] substitution as the least effective means. The transition from ferroelectric to relaxor behaviour occurs between 20%¿30% homovalent Zr substitution. In the heterovalent case, a Nb concentration between 5%¿7% is already sufficient to cause the same effect. In the case of co-substitution, a concentration of 2.5% Nb and 20% is sufficient to effectively disrupt any ferroelectric long-range order. In general, the measurements imply that even though the highest substituted systems can be considered as being in the fully disordered relaxor state with ergodic polar nanodomains, traces of classical ferroelectric behaviour in the form of hysteresis loops recorded by PFM can still be obtained. Furthermore, charge-balancing vacancies significantly influence image acquisition by PFM in systems with heterovalent substitution, leading to non-negligible, non-piezoelectric signal contributions and even material swelling on the surface upon poling. This thesis corroborates that PFM is an effective method for determining the ferroelectric properties of relaxors, provided that it is complemented by additional experimental methods like Raman/dielectric spectroscopy, X-ray diffraction or electric Polarisation-Field loop measurements.