A microelectromechanical systems (MEMS)-based environmental monitoring platform was presented in this dissertation. All devices were realized using thin-film piezoelectric-on-substrate (TPoS) technology, which provides a path to integrate various functionalities on a single substrate with MEMS components. TPoS resonators exhibit high quality factors (Qs) in air and are capable of low-power oscillator implementation, which further qualifies such a platform for mobile and portable systems. To validate the TPoS platform, gravimetric humidity sensing was demonstrated with thermally-corrected output by an uncoated "reference" temperature sensor. Also presented were TPoS sensors for toluene and xylene, which are pollutants of great importance for indoor and outdoor air quality as well as health screenings. Silicon dual-mode resonators and oscillators for self-temperature sensing were also explored. Dual-mode thermometry exploits the inherent frequency-temperature dependence of silicon to accurately and locally measure device temperature, forming an essential building block of highly stable oscillators and sensors. Multi-axis piezo-on-Si kinetic energy harvesting (KEH) devices with integrated frequency-upconverting transducers were also introduced. Devices were micromachined on the same substrate as TPoS resonant sensors and have an individual volume in mm3, enabling applications in wireless autonomous sensor nodes. In remote locations where continuous operation may be required, TPoS energy harvesters can provide battery replacement or recharging alternatives that do not increase overall system size.