We give new examples of noncommutative manifolds that are less standard than the NC-torus or Moyal deformations of R n . They arise naturally from basic considerations of noncommutative differential topology and have non-trivial global features.The new examples include the instanton algebra and the NC-4-spheres S 4 θ . We construct the noncommutative algebras A = C ∞ (S 4 θ ) of functions on NCspheres as solutions to the vanishing, ch j (e) = 0 , j < 2, of the Chern character in the cyclic homology of A of an idempotent e ∈ M 4 (A) , e 2 = e , e = e * . We describe the universal noncommutative space obtained from this equation as a noncommutative Grassmanian as well as the corresponding notion of admissible morphisms. This space Gr contains the suspension of a NC-3-sphere intimately related to quantum group deformations SU q (2) of SU(2) but for unusual values (complex values of modulus one) of the parameter q of q-analogues, q = exp(2πiθ).We then construct the noncommutative geometry of S 4 θ as given by a spectral triple (A, H, D) and check all axioms of noncommutative manifolds. In a previous paper it was shown that for any Riemannian metric g µν on S 4 whose volume form √ g d 4 x is the same as the one for the round metric, the corresponding Dirac operator gives a solution to the following quartic equation,where is the projection on the commutant of 4 × 4 matrices. We shall show how to construct the Dirac operator D on the noncommutative 4-spheres S 4 θ so that the previous equation continues to hold without any change. Finally, we show that any compact Riemannian spin manifold whose isometry group has rank r ≥ 2 admits isospectral deformations to noncommutative geometries.