Raubenheimer, T.O. JACoW Geneva, Switzerland The effect of ions are evaluated in LCLS and LCLS-II. X-ray FEL's require high-brightness beams, that generate very high internal beam fields which can potentially lead to tunneling ionization. The FEL performance is sensitive to betatron mismatches which can arise from the focusing due to ions. In LCLS, both collisional and tunneling ionization may have an impact on single electron bunches while LCLS-II will have the additional complication of trapped ions in the MHz-rate multi-bunch trains. Analytic and simulation results for the LCLS and LCLS-II are presented and scalings are described to estimate the effects in other X-ray FEL's.