Nanotechnology is emerging as a new technological revolution concerned with fabricating things, typically materials and devices at molecular or atomic level. Nanocarriers are nanosized materials widely used as delivery systems to transport therapeutic agents to targeted sites in the body. The effectiveness of the treatment can be improved by the usage of various nanocarriers such as polymeric nanoparticles, lipid-based carriers, gold nanoparticles, dendrimers, carbon tubes, etc. The choice of employing such carrier systems is dependent on the attributes (e.g., charge, size, solubility, etc.) of the bioactive substances, efficacy and safety of the bioactive-carrier complex, together with its application and the administration method of the complexes to the systemic circulation. This chapter may aid in providing a basic understanding of different types of nanocarriers, their applications, preparation techniques, and evaluation techniques, and of various nanocarriers targeting the brain and some natural products in nanocarriers.