This paper studies the depth of noisy decision trees in which each nod
e gives the wrong answer with some constant probability. In the noisy
Boolean decision tree model, tight bounds are given on the number of q
ueries to input variables required to compute threshold functions, the
parity function and symmetric functions. In the noisy comparison tree
model, tight bounds are given on the number of noisy comparisons for
searching, sorting, selection and merging. The paper also studies para
llel selection and sorting with noisy comparisons, giving tight bounds
for several problems.