The Internet of Things (IoT) is an infrastructure of physical objects connected via the Internet that can exchange data to achieve efficient resource management. Billions of devices must be self-powered and low-cost considering the massive scale of the IoT. Thus, there is a need for low-cost ambient energy harvesters to power IoT devices. It is a challenging task since ambient energy might be unpredictable, intermittent and insufficient. For example, solar energy has limitations such as intermittence and unpredictability despite utilizing the highest power availability and relatively mature technology. Designing a multi-source energy harvester (MSEH) based on continuous and ubiquitous ambient energy sources might alleviate these issues by providing versatility and robustness of power supply. However, combining several energy harvesters into one module must be done synergistically to ensure miniaturization, compactness and more collected energy. Also, additive manufacturing techniques must be used to achieve low-cost harvesters and mass manufacturability. This dissertation presents two different kind of ambient energy harvesters, namely radio frequency energy harvester (RFEH) and thermal energy harvester (TEH). Each harvester is individually optimized and then synergistically combined into a MSEH. First, RFEH is designed for triple-band harvesting (GSM900, GSM1800, 3G2100) using the antenna-on-package concept and fabricated through 3D and screen printing. TEH collects energy from temperature fluctuations of ambient environment through a combination of thermoelectric generators and phase change materials. It is adapted specifically for the desert conditions of Saudi Arabia. Later, TEH and RFEH are combined to realize MSEH. Smart integration is achieved by designing a dual-function component, heatsink antenna, that serves as a receiving antenna of RFEH and a heatsink of TEH. The heatsink antenna has been optimized for both antenna radiation performance and heat transfer performance. Field tests showed that the MSEH can collect 3680μWh energy per day and the outputs of TEH and RFEH have increased 4 and 3 times compared to the independent TEH and RFEH respectively. To validate the utility of the MSEH, a temperature/humidity sensor has been successfully powered by the MSEH. Overall, sensor’s data can be wirelessly transmitted with time intervals of 3.5s, highlighting the effectiveness of the synergistic MSEH.