This thesis gives a theoretical description of the active tilting-pad journal bearing (ATPJB). It provides the qualified reader with the tools to model an ATPJB, while staying clear of pitfalls. The model is based on well known techniques and allows for local stability analyses, harmonic stationary analyses as well as nonlinear time domain analyses of ATPJBs. Examples of all of these analysis types are presented. As opposed to some of the more convoluted modelling methods in the literature, the presented one permits dimensional analysis in a straight forward and intuitive manner. Dimensional analyses are performed for a tilting-pad journal bearing (TPJB) and an ATPJB under static conditions, followed by a generalisation to dynamic conditions (transient as well as stationary harmonic). These analyses will be of interest to experimentalists, since they permit experiments performed on scaled down test rigs to be extrapolated to the full size product in a confident manner. Finally, a selection of simulation results is presented. These prove that ATPJBs show promise in attenuating some of the limitations of TPJBs. But also, they show that ATPJBs should be modelled, tuned and implemented with great care, since the addition of a control system may just as well harm rotor-bearing performance as improve it.