Biochar incorporation into consumer products could help accelerate the transition to a circular economy. One way to do this is by mixing biochar with plastics, both conventional and biodegradable. In the last five years, RIT’s research on biochar composites has demonstrated the effects of biochar feedstock, particle size and polymeric matrix on the performance, processability and sustainability of the material. Biochar (BC) can be used as a reinforcement filler, processing additive, opacifier agent (to control light transmission), and pigment in a wide range of plastics. The relationship between BC feedstock and the polymer matrix has been investigated through mechanical, thermal, and morphological characterization. For example, three different polymeric matrices (i.e., polypropylene, polycaprolactone and polylactic acid) were filled with two different BC feedstocks (i.e., dairy manure, and wood chip), thus illustrating a the synergy between specific feedstocks and the polymer matrix, and indicating that wood-based BC is a more suitable filler for hydrolysis-sensitive resins like polylactic acid (PLA). The effect of waste coffee ground BC on the thermoformed properties of a biodegradable polycaprolactone/starch composite was investigated as well. The samples showed effective thermoforming up to 20% BC by mass. A factorial analysis was conducted to quantify the effect of coffee BC content and particle size on the mechanical properties of polylactic acid- starch composites. Smaller particle size at low concentrations showed enhancement in composite strength and toughness. On the other hand, coarser particles are better for high loadings where cost reduction is a commercial priority. Additionally, biochar was effectively used as an anti-blocking agent in thin films made through the blown film process. We assessed the sustainability of BC as an opacifier alternative in agricultural mulch films. Cradle-to-gate life cycle assessment and techno-economic analysis (LCA/TEA) coupled with mechanical characterization showed that performance limitations require additional film thickness, thus hindering their economic competiveness and environmental benefits. In general, BC dispersion in polymer matrices continues to be a challenge in engineering applications because of the relatively large size of the particles relative to common carbonaceous fillers such as carbon black.  In conclusion, our work demonstrates that biochar can be a sustainable filler for plastic composites, but commercially relevant products containing biochar will require the right match of feedstock, pyrolysis conditions, particle size reduction, polymer matrix, and an economically viable value chain.