On stability of linear neutral differential equations with variable delays

We present a review of known stability tests and new explicit exponential stability conditions for the linear scalar neutral equation with two delays $$\dot{x}(t)-a(t)\dot{x}(g(t))+b(t)x(h(t))=0,$$ where $$|a(t)|<1,~ b(t)\geq 0, ~h(t)\leq t, ~g(t)\leq t,$$ and for its generalizations, including equations with more than two delays, integro-differential equations and equations with a distributed delay.