We present a brief survey on the state of the theory of the real analytic regularity (real analytic hypoellipticity) for the solutions to sums of squares of vector fields satisfying the Hörmander condition. Contents 1. Introduction: the C ∞ hypoellipticity 1 2. The real analytic case: a short history, examples and counterexamples 5 3. Geometry of the characteristic variety: stratifications and the Treves conjecture 10 3.1. The analytic stratification 11 3.2. The symplectic stratification 12 3.3. The Poisson stratification 13 4. Examples and counterexamples 15 4.1. Examples 16 4.2. Counterexamples 18 5. Open problems 28 5.1. The 2 dimensional case 28 5.2. The 3 dimensional case 30 5.3. The case of dimension n ≥ 3 33 References 34