This is a Phase II, randomized, double-blind, placebo-controlled, cross-over POC study of stable patients with Schizophrenia or Schizoaffective disorder. The primary objective of this study is to test the efficacy of treatment with one of two does of a glycine transporter inhibitor (GlyT1I) combined with cognitive remediation to enhavce cognitive function. Subjects will be randomized to one of two doses of the glycine transporter inhibitor (GlyT1I) and placebo twice daily in addition to their antipsychotic medication for 2 treatment periods, each lasting a minimum of 5 weeks. Treatment periods will be separated by a washout period lasting approximately 3 weeks.