Cellulose is one of the most versatile biopolymers found in nature. Its use is now transitioning from sustainability-focused development to provide technical solutions across a wide range of applications. This review paper describes the use of cellulose as reinforcement in biocomposites and recent related advances in 3D printing technologies, including 4D (responsive/smart) printing and current/future applications. Relevant aspects such as the origin and intrinsic/structural properties of cellulose and cellulose/matrix interaction during processing are discussed. A particular focus is directed to identify opportunities for the development of new cellulose-based composites, formulation requirements for 3D/4D printing, and comprehension of the interplay between rheology, chemistry, and processing. Overall, new materials and technologies based on cellulose have shown promising results for real-life applications, opening new scientific development opportunities and a new generation of sustainable and advanced materials.