Fibre optic sensors have shown significant potential in commercial sensor systems due to their key characteristics such as small size, light weight and immunity to electromagnetic interference. Fibre grating-based sensors have gained considerable attention in recent years among all the other fibre optics sensing systems. The fibre Bragg grating (pBG) is a one such grating-based fibre sensor and has been widely used in the sensing areas such as temperature, strain and relative humidity. The long-period grating (LPG) is a more recently developed type of fibre grating which is being investigated and used as sensors and has until now very much taken 'second place' to the FBG sensors. Its higher sensitivity to temperature when compared to the FBG and its special feature that is its sensitiven.ess to the external refractive index has attracted a lot of interest in realising LPG-based sensing systems. This thesis describes two types of LPG, sensor systems which were developed and investigated for monitoring temperature and humidity. In the first, the LPGs written in different fibres were investigated for potential LPG based temperature sensors. In order to get a clear understanding, the temperature investigation has been carned out in standard silica fibre, B-Ge photosensitive fibre and specialist fibres (Bi-Ge, Sn-Er-Ge and Sb-ErGe) for elevated temperature measurements. In addition to this further investigation has been carned out on an LPG written in B-Ge photosensitivity fibre to identify the highest temperature sensitive resonance band among all, based on a theoretical study which has been reported by researchers recently. Secondly, a number of LPG-based humidity sensors have been developed and investigated. Polyimide (PI) and polyvinyl alcohol (pVA) coated LPG sensors were developed and subjected to a series of tests. PI coated sensors have not shown any response when exposed to various humidity conditions. In contrast, the PVA coated sensors have shown good response when exposed to various humidity conditions and detailed experiments were carned out on PVA-coated LPG sensors ofvarying thickness and summary ofconclusions reported.