Quasi charge neutrality is one of the most elementary concepts in plasma physics that is discussed early in many introductory plasma physics textbooks based on the physics of Debye shielding and plasma oscillations. However, when nonlinear effects are considered, electrostatic structures in steady state with spatial scales larger than the Debye length can exist in a plasma. In fact, electrostatic structures have been observed in many regions of space plasmas, including the solar wind, the magnetosphere, the auroral acceleration region, and in association with shocks, turbulence, and magnetic reconnection. In this talk, I will introduce one possible theoretical description of some of these structures --- Bernstein-Greene-Kruskal (BGK) modes, which are exact nonlinear solutions of the Vlasov-Poisson system of equations in collisionless kinetic theory. In particular, I will present recent Particle-in-Cell (PIC) simulations of 2D BGK mode solutions in finite magnetic field found analytically a few years ago [Ng, Bhattacharjee, and Skiff, Phys. Plasmas 13, 055903 (2006)] to study their stability.