This thesis investigates the development of new synthesis methods and structures of analogue filters considering two types of active elements: operational transconductance amplifiers and current conveyors. Starting from an nth-order generic transfer function, it is shown how an analytical synthesis approach can be used to generate systematically filters that have less component count than previously known filters whilst maintaining the desirable features of grounded passive components and the employment of single-input active devices. Two new synthesis methods for current-mode filters and two new synthesis methods for voltage-mode filters have been developed including their associate structures to provide nth-order filtering functions. Equal transconductance and equal capacitance appear in the voltage-mode synthesized filter structures particularly suitable for VLSI implementation. Simulation results of various filter circuits obtained from the proposed synthesis methods are given based on CMOS design of the active elements. Despite the numerous publications reporting filter biquads based on transconductance amplifiers and current conveyors, it is shown how such biquads normally make trade-offs with respect to some key performance parameters including excessive number of passive and active elements and the need to impose component choice conditions in order to provide universal filtering. This includes low-pass, high-pass, band-pass, notch and all-pass functions. This thesis proposes the concept and design of 'unified' filter model based on nullators, norators, current mirrors, resistors and capacitors, from which two new biquad implementations based on two different active elements have been developed. It is shown that such biquads can achieve five key performance parameters simultaneously and without trade-offs. Finally, a comparative analysis between the various synthesis methods and filter structures reported throughout the thesis has been carried out with the general aim of assisting the designer to develop the appropriate filter circuit given a filtering problem and an implementation style.