The problem considered is estimation of the total value of a character for a finite population from a sample when the units are selected with varying probabilities without replacement. Several unbiased estimators are proposed. Exact expressions and unbiased estimators for the variances of the estimators are obtained. Certain properties of Yates and Grundy's estimator of variance are proved. Examples are included to study the relative performance of the various estimators. The results obtained for unistage sampling in the first part of the paper are extended to multistage designs in the second part.