This research entailed the development of resonant dielectric sensors that utilising alternating electric fields at microwave frequencies. Characterisation and identification of biological samples is currently an expensive and time consuming procedure due to the extreme variation of seemingly similar biological systems. The work describes the development of resonant dielectric sensors for five distinct applications. The aim of the research was to develop non-invasive, affordable and compact dielectric spectroscopy sensors that may be used in-situ to characterise organic and biological systems. The suitability of using dielectric spectroscopy to a number of applications has been investigated. This is to address disadvantages of conventional laboratory analysis such as lengthy, costly and labour intensive assessment methods that require intermittent sampling and/or off site analysis. The research is structured into a series of progressive stages (work packages) that integrate and culminate into feasible inline analytical procedures that can quickly, safely and inexpensively assess a specified variable in organic and biological systems. Overall, a number of sensors using electromagnetic radiation at microwave frequencies were developed that have proven to be sensitive and extremely versatile to a number of detection and monitoring applications. Miniaturisation of microwave generation and analysis circuits will enable the sensors to be incorporated into feasible compact devices for in situ analysis of the analyte in question.