The index of subcriticality is used to quantify the degree of “how deep a target system has a margin below the critical state.” There are three main categories of subcriticality measurement techniques: the static method, the kinetic method, and the reactor noise analysis method. This chapter is dedicated to explaining the following methods that can be carried out at the Kyoto University Critical Assembly: the neutron source multiplication method, the inverse kinetics method, the pulsed neutron source method, and the reactor noise analysis method such as the Feynman- \(\alpha\) method. As described in this chapter, each of the subcriticality measurement techniques has both advantages and disadvantages. Through the explanations of the principles of the measurements and the procedures, this chapter helps readers understand how to properly select the various subcriticality measurement techniques depending on an experimental condition for a target system.