The identification problem of decentralised control systems (DS) is considered. Analysis shows that this problem has not been given sufficient attention. The complexity of systems and a priori uncertainty require the development of approaches and methods. DS parametric identifiability (PI) requires a solution. We propose the approach to the PI assessment based on the fulfilment of the constant excitation condition and consider relationships in the subsystems. PI conditions are got and algorithms for parametric and signal adaptive identification are received. We consider DS with non-linearities satisfying the quadratic condition. The exponential dissipativity of the identification system is proved using Lyapunov vector functions. The influence of interrelations considers on properties of parameter estimates. Examples are given. A method is proposed for the construction of adaptive algorithms under functional constraints.