Affordable, sensitive, selective, fast-responding, label-free sensors are currently in high demand for many of today's applications and technologies, particularly in the food industry, bio-sensing applications, and quality control. In addition, modern technologies such as a lab-on-a-chip involve microfluidic analysis, which requires highly accurate and miniaturized sensing systems. These systems can be implemented in biomedical applications such as point-of-care diagnostics, as well as in environmental monitoring, agriculture, biotechnology, and public health and safety. A need, therefore, exists for highly accurate and reliable sensing systems that can meet the requirements of these applications. This dissertation presents electrically-small planar microwave resonators for the design of near-field sensors that can satisfy the needs of the aforementioned applications. This thesis proposes a number of novel concepts related to miniaturization and the enhancement of the sensitivity of electrically-small sensors. In the first part of the thesis, an analysis of the sensitivity of complementary split-ring resonators (CSRRs) with respect to changes in resonator topology is presented. Eigenmode solvers, circuit models, numerical simulations, and laboratory measurements were all employed for the analysis. The results show that the resonance frequency is adjustable and scalable. The second part of the thesis proposed an ultrasensitive near-field sensor for detecting submillimeter cracks in metallic materials. Experimental measurements revealed that a surface crack of 200 um wide and 2 mm deep results in a 1.5 GHz shift in the resonance frequency. The results led to the idea of utilizing CSRRs for designing near-field sensors for crack detection in dielectric materials. The work was further extended to increase the sensitivity of planar CSRRs to detect the presence of dielectric materials. This concept is based on increasing the sensing areas per unit length and on the utilization of multiple, identical, and coupled resonators. Although the electromagnetic energy stored in electrically-small planar resonators is concentrated primarily in an electrically-small volume, most of that energy is located in the host substrate, thus limiting the sensitivity required for detecting changes in the material under test (MUT), which differs from the host substrate. For this reason, a sensor designed for enhancing the EM energy stored in the sensing volume that is exposed to the MUT is proposed. The design concept is based on the use of a three-dimensional capacitor. For validation purposes, a complementary electric-LC resonator (CELCR) and two metallic bars were utilized for designing the sensor for dielectric materials. Furthermore, by adopting the concept of three-dimensional capacitors, microwave sensors based on planar SRRs are introduced in order to 1) enhance the sensitivity, 2) allow for flexible tunability, and 3) create novel sensors for fluidic applications. For validation purposes, an SRR-based sensor was designed and tested using numerical simulation and experiments to detect fluid materials and fluid levels. The SRR with the three-dimensional capacitors was also utilized to design probes for the near-field scanning microscopy. An additional component of this research was, therefore, an exploration of the miniaturization of CELCR sensing areas so that these devices could be loaded with three-dimensional capacitors in order to design a sensitive near-field sensor for microscale-based technologies. The ability of the sensor to detect the presence of magnetic materials was also investigated numerically. For applications in which flatness or compactness is a relevant factor, enhancing sensitivity with the use of three-dimensional capacitors is not an ideal solution. Although classical planar antennas such as patch antennas are subject to a lack of EM energy localization in small areas, the adoption of the split concept, utilized in electrically-small resonators, can improve these antennas for use in designing near-field microwave sensors. This thesis proposed a planar microwave sensor based on an annular ring resonator loaded with a split, thus enabling it to operate at lower frequencies and to enhance the quality factors. The sensor was tested experimentally with respect to characterizing dielectric slabs and detecting the presence of fluidic materials. The last part of the thesis introduced the concept of an intelligent sensing technique based on the modulation of the frequency responses of near-field microwave sensors for the characterization of material parameters. The concept is based on the assumption that the physical parameters being extracted are uniform over the frequency range of the sensing system. The concept is derived from the observation of the sensor responses as multidimensional vectors over a wide frequency range. The dimensions are then considered as features for a neural network. The concept has been demonstrated experimentally for the detection of the concentration of a fluid material composed of two pure fluids.