Hierarchical nanostructure sensors are demonstrated and discussed in this chapter. Firstly, hierarchical nanostructures suitable for sensors are introduced and classified depending on hierarchy and morphology, followed by their effective preparation methods. Gas sensors, biosensors and surface enhanced Raman spectroscopy (SERS) sensors that exploit hierarchical nanostructures are presented in detail and evaluated in terms of sensitivity/resolution and response time. Hierarchical nanostructure sensors are proven to possess higher performances, such as sensitively, resolution and response time, than their compact and dense counterparts. They are very promising for the detection of ions, molecules and biomarkers at trace-level due to their high sensitivity and toxin, hazardous gas and biochemical weapons due to their fast response characteristics.