Carbon nanotubes (CNTs) are hollow graphitic nanostructures with unique physical properties and the ability to encapsulate foreign materials inside their cores, forming hybrid nanostructures called filled CNTs. Filling CNTs improves their mechanical strength and combines the properties of both CNTs and filled materials. Due to their high surface area and quantum confinement effects, filled CNTs exhibit enhanced electrochemical properties. Various inorganic and organic materials can be encapsulated into CNT cavities, enabling applications in batteries, cancer therapy, nanoreactors, and catalysis. This article reviews recent advances in the synthesis, properties, and applications of filled CNTs along with their future aspects.