This chapter focuses on the processes involved in compression and injection molding. It explains the differences between thermoplastic polymer composites and thermosetting polymer composites, which are processed by compression as well as same for which are processed by injection molding. The differences in various factors like mechanical properties, processability, cost of production, etc., for both the molding techniques are stated and elaborated. The experimental values comparing the mechanical properties of the injection and compression molding technique are tabulated to exhibit the differences. The findings demonstrated that the manufacturing techniques utilized to make the composites have a big impact on their mechanical properties. For this, various fiber-reinforced composites consisting of various matrices and different fibers materials are compared. Both natural fibers like sisal and agave as well as artificial fibers like glass and carbon fiber are used to exhibit the difference between injection molding technique and compression molding technique. The mechanical properties that vary in both techniques are distinguished and explained.