WIRELESS COMMUNICATIONS have undergone a drastic development in the past decade and have also been an important motivator for small antenna research. Introduction of new communications systems with wider system bands, evolution from single-band to multiband standard and a significant decrease in average of the terminal size are on the roadmap of the industry. All this combined with strict limitation due to the protection of mobile devices users from radiation exposure has created needs for adequate antenna solutions and better understanding of small antennas on small complex platforms. As answer to these great challenges, a purposeful investigation of a new concept for small antennas development on small platforms is proposed in this thesis. The performance of a small antenna mounted on a small conductive object, such as the chassis of a mobile phone is strongly affected by the size and shape of the object as well as the position of the small antenna on it. For a better understanding of the radiation properties of the radiating system constituted by a small antenna and the conductive chassis of a mobile phone device, this thesis conceptually considers the small antenna and the conductive chassis as two distinctive entities and separately investigates the radiation properties of the metal chassis. The idea here is that from the knowledge of the chassis radiation properties, one easily identifies the optimal location of the small antenna on the chassis and achieves a design of the small antenna to optimally meet some given requirements. Investigation of the conductive chassis radiation properties is based on the theory of characteristic modes. In fact, the theory of characteristic modes is a powerful analytical concept comparable to what modal analysis represents for waveguide circuits. At a frequency of interest this concept provides an insight on the contribution of different characteristic modes in the overall chassis radiation properties, which inexorably leads to the knowledge of the dominant characteristic mode. Resonant characteristic modes which dominates the radiation properties at their respective frequencies assign their radiation properties expressed in terms of resonance frequency and radiation quality factor to the conductive chassis. In the frame of this work, a numerical evaluation of characteristic modes on the surface of conductive chassis of mobile phones is proposed. This has been made possible through the solution of the resulting generalized eigenvalue problem representing the radiating system and leading to characteristic modes. For the resolution of the resulting generalized eigenvalue problem, an algorithm based on the QZ-decomposition is conceived and a corresponding software is developed. As application, results on the radiation properties as well as optimal location of the small antenna (considered as an exciter or coupler) to achieve an effective coupling with the chassis is presented for generic bar-type and folder-type phones. For the latter two cases of operation have been distinguished: the closed and open states. A simplified version of the theory of characteristic modes for the analysis of chassis radiation properties is presented in this thesis. It is based on the far-field radiation properties of a small radiator in free-space. The radiation vector is approximated by moments of different orders depending on the shape of the chassis. This practical approach operates by post-processing output data from standard commercial electromagnetic softwares. The purposeful utilization and the tuning of chassis modes for the design of multiband radiation characteristics on generic bar-type and folder-type geometries is demonstrated.