I discuss primordial fluctuations and its non-Gaussianity in
models with isocurvature perturbations. First I describe a general
formalism which can give a systematic calculation of non-linear
perturbations in models with isocurvature fluctuations. Then I discuss isocurvature
fluctuations and non-Gaussianity in the mixed curvaton and inflaton
model where the curvaton and the inflaton are both responsible for
cosmic density fluctuations. The issue of differentiating CDM and
baryon isocurvature modes using 21 cm fluctuations is also briefly
discussed.