Positive temperature coefficient of resistivity (PTCR) materials have become very important components, and among these materials barium titanate compounds make up the most important group. When properly processed these compounds show a high PTCR at the Curie temperature (the transition temperature from the ferroelectric tetragonal phase to the paraelectric cube phase). In the first half of this paper literature related to the resistivity-temperature behaviour is discussed. As explained by the well established Heywang model, the PTCR effect is caused by trapped electrons at the grain boundaries. From reviewing experimental results in the literature it is clear that the PTCR effect can not be explained by assuming only one kind of electron trap. It is concluded that as well as barium vacancies, adsorbed oxygen as 3d-elements can act as electron traps. In the second half of this paper, the influence of the processing parameters on the PTCR related properties is discussed. Special emphasis is placed on the phenomenon that the conductivity and grain size decrease abruptly with increasing donor concentration above similar to 0.3 at%. Several models explaining this phenomenon are discussed and apparent discrepancies in experimental data are explained.