Fused Deposition Modeling (FDM) is one of the most commonly utilized low-cost and promising processes included in the category of Additive Manufacturing (AM) that employs polymer-based filaments as feedstock materials to fabricate physical objects through a layer-by-layer deposition process. FDM has already found its possible applications in several industrial fields due to its safe and simple fabrication process, inexpensive machinery, as well as low maintenance and material costs. Although the FDM process has presented enormous progress over the last two decades, there are still challenges regarding the quality, consistency and repeatability of the printed parts that dramatically impede the technology’s broad adoption and employment for the manufacturing of finished components ready for use. Most of the process defects in FDM built parts are strongly associated with the nature of the process itself. In particular, the inherently developed temperature variations, non-uniform thermal gradients and the process induced residual strains within the FDM parts are of utmost importance, since these parameters significantly affect the final structures’ quality and overall performance. In the scope of optimizing the FDM technique for achieving higher levels of part quality and repeatability, the development of accurate and reliable sensing methods and tools for in-situ process monitoring and for effective quality control of the printed structures is increasingly demanded. This topic poses a great challenge, while is considered as a high priority area for research. The current thesis work aims to shed light on this challenging topic through the development of a sophisticated and reliable sensing method, predominantly involving single-point and multiplexed fiber Bragg gratings (FBGs), as well as temperature measuring sensors for in-situ and real-time monitoring of the temperature profiles and the residual strains developed within pure thermoplastic parts and continuous fiber reinforced thermoplastic composites (CFRTPCs) fabricated via the FDM technique. The embedded FBG sensors are also used for identifying the post-fabrication induced strain magnitudes at the free-standing state of the structures and for measuring the host materials’ coefficients of thermal expansion (CTEs), when the parts are subjected to thermal cycling tests. More specifically, the specimens built in the present work are divided into three main categories: 1) Pure thermoplastic blocks, 2) Pure thermoplastic square thin plates and 3) Continuous fiber reinforced thermoplastic composite (CFRTPC) blocks. A methodology is developed and followed for the sensors’ accurate embedment within all considered FDM-fabricated test samples, whose steps are described in detail in Chapter 4 of the thesis. As far as the thermoplastic block samples are concerned, the effect on the developed strains as a result of the specimens’ position onto the build platform and the optical fibers’ embedment direction within the parts is also investigated. Additionally, a methodology is presented for simultaneous monitoring of strain and temperature profiles from the recorded spectrum of the embedded optical fibers, when the deposited material remains close to its glass transition temperature and it is not exposed to steep temperature changes. The post-fabrication induced strain magnitudes are determined based on intra- and inter-layer wavelength measurements derived by FBG sensors integrated within the block samples’ 3rd or 21st layer and within the 3rd, 21st and 39th layers, respectively. The results indicate that the parts’ build orientation onto the platform affects the temperature profiles developed during the printing procedure and consequently, the in- and post-process induced residual strains. Significant strain magnitudes are generated within the specimens during the fabrication process, while the inter-layer wavelength measurements obtained at the samples’ free-standing state indicate the presence of warpage. With respect to the 3D printed thermoplastic square thin plates, an array of single-point and multiplexed optical fibers as well as thermocouple sensors is integrated within the parts’ midplane, allowing for real-time mapping of the process induced temperature profiles and strain variations throughout the whole FDM fabrication process. The influence of the Bragg gratings’ embedment direction on the calculated residual strains is also investigated through the incorporation of gratings in different orientations (i.e. 0°, +45° and 90°) within positions located at the same plate regions. Based on the obtained results uniform temperature profiles are displayed in the structures’ middle section, while maximum temperature values are recorded at locations close to the plates’ corners. Moreover, the conducted experiments demonstrate the FBG sensors’ ability to determine the in-and post-process induced residual strains. The effect of fiber orientation and type on the induced temperature profiles, residual strains and thermal expansion behavior is identified for the 3D printed CFRTPC block samples. Significant information is obtained about the composites’ thermal expansion behavior based on the wavelength measurements recorded by the integrated FBG sensors during thermal cycling testing. Finally, it is shown that the fiber reinforcement within the FDM-fabricated composite parts significantly reduces the in- and post-process induced residual strain magnitudes, when compared to the corresponding ones calculated for the pure thermoplastic block specimens.