This chapter highlights the nanocellose-based composite sensors as powerful tools in multiple sensing/detecting applications and polymeric matrixes. The worldwide need for rapid, precise/accurate, selective/specific detection positions the nanosensors as crucial devices for sensing purposes. The authors discuss the basic sensor conception, a basic architecture applied in different spatial scales and research areas, such as environmental sciences, medicine, and optoelectronics. Chemo- and biosensors are also described in the chapter. The use of nanocellulose as supporting or embedding material in sensors and polymeric matrixes gives the attributes of this noble material, which, combined with active and sensitive materials at the nanoscale, assemble new opportunities in the sensors field.