Radio frequency identification (RFID) is an auto-identification technology realised by radio waves. The ultimate goal of RFID is the item-level tagging of all kinds of products in supply chains. This goal challenges industry and academia in many aspects. Passive UHF RFID systems, when compared with other RFID systems, are believed to possess advantages in achieving that goal. However, UHF RFID systems possess two serious disadvantages: (i) the relatively large antenna size, and (ii) the sensitiveness to the metallic items on which a tag is mounted. Those two deficiencies make a large number of small size objects and metallic objects hard to tag. In addition, different applications also bring special requirements or limitations in adopting UHF RFID systems, such as in the case of a container seal, the requirement for tags to have a physical security function, and in other cases such as pallet shipping, the requirement for detecting massive numbers of items densely stacked together. Finally, of course, cost is one of the key limitations if one intends to apply his or her design down to item-level tagging commercially. Hence each of the inherent deficiencies of the system itself and the limitations caused by the application, or a combination of all or some of the deficiencies and limitations make a large number of items hard to tag and impedes the item-level tagging target. The research in this thesis aims, by antenna design and electromagnetic wave analysis, to provide feasible and affordable solutions for some of those hard-to-tag objects in UHF RFID systems, and the thesis can be divided into five parts. In detail, the first part of the thesis gives the motivations, contributions and structure of this thesis. In addition it also provides a brief introduction to RFID systems and about how they are operated, developed, classified, regulated and standardised. The second part of this thesis presents basic terminologies and design criteria in tag antenna design, transponder IC design and reader design. Factors which limit the operating range of UHF RFID systems are discussed. Following this discussion, a novel method making use of a scattering matrix for evaluating the operating range of a UHF RFID system deployed in an arbitrary environment is proposed. In the third part, concerning the meander line dipole antenna (MDA), one of the approaches to minimising tag antenna size is analysed in terms of its resonant frequency, size reduction contributors, radiation pattern and efficiency. An analytic formula for calculating the resonant frequency of an MDA on a dielectric substrate as an RFID tag antenna is established. Based on the analysis, a novel tag antenna with a physical security function (an electronic seal) for protecting shipping containers was designed and experimentally verified. The fourth part of this thesis puts emphasis on metallic item detection. The reason of why common dipole based tag antennas cannot work well in close proximity to metal is given. Previous solutions and their own demerits in solving this problem are summarised. Then, a low profile, simple structure, compact size solution is introduced via the artificial magnetic conductor concept. Furthermore, a general DVD disc contains a very thin metal layer inside for the purpose of reflecting laser. That layer may not bring many troubles in identifying a single DVD by a UHF RFID system, but if thousands of DVDs were stacked, the role the metal component plays in degrading the detection of each DVD in the stack should be investigated. An approach in detecting a large number of DVDs (up to 2000) densely stacked is thus presented. Conclusions of the work in this thesis are drawn as the last part of the thesis. Besides conclusions the last part also includes some recommendations for future work and the description of the original contributions of this thesis. The potential benefits of item-level tagging in supply chains are enormous. The existence of a large number of hard-to-tag objects is one of the main challenges in achieving item-level tagging. The studies in this thesis extend the scope of the detectable objects and this extension makes item-level tagging more realisable.