Circles are among the most frequently tested topics in JEE coordinate geometry, appearing as 1-2 questions annually. The chapter covers equations of circles, tangent and normal lines, chord properties, and the interaction of two or more circles. The standard form (x-h)^2+(y-k)^2=$r^2$ and general form x^{2+y}^{2+2gx+2fy+c}=0 are the foundation. The T=0 substitution rule is the single most powerful technique: it unifies the tangent at a point, chord of contact, chord with given midpoint, and pair of tangents into one framework. Two-circle problems — involving radical axes, common tangents, orthogonality, and coaxial systems — form the advanced portion. JEE problems typically test tangency conditions, length of tangent from an external point, and the family of circles through intersection points. The topic connects strongly to conics (parabola, ellipse, hyperbola) where similar T=0 techniques apply.