The structure and stability of thin one-dimensional current sheets in which the ions carry most of the current and momentum are investigated. It is demonstrated that for the case of a cold current sheet the characteristic half width &lgr; is given by &lgr;≈(Bz/B0)4/3c/&ohgr;p0, where Bz is the normal field component (assumed to be spatially uniform), B0 is the asymptotic magnitude of the reversing field, and c/&ohgr;p0 is the collisionless ion skin depth based on the asymptotic density. The trapping of the ion orbits in the reversal region leads to a strong density enhancement at the center of the current sheet given by nc/n0≈c/&lgr;&ohgr;p0≈(B0/Bz)4/3. A two-dimensional self-consistent dynamical simulation model is developed which demonstrates that these idealized current sheets are unstable to kink perturbations driven by the anisotropic pressure distribution produced by the chaotic nature of the particle orbits in a field-reversal region. The combined effects of chaotic scattering and finite thermal spread are likely to preclude the formation in the geomagnetic tail of stable current sheets with half width less than about 0.25c/&ohgr;p0(~200 km). ¿ American Geophysical Union 1992