This book is designed as an advanced guide to mathematical methods in science. It covers many topics of practical Statistics: Descriptive Statistics: frequency tables for grouped and ungrouped data, position statistics: arithmetic mean, median, mode, quartiles and deciles, dispersion statistics: variance, standard deviation, coefficient of variation, and mean deviation, statistical tables and summary, histograms and frequency polygons, Chebishev's Theorem and bias; the method of least squares for a polynomial fit (regression analysis), correlation coefficient, application problems. It is included the derivation of many formulas to clarify the concepts of Descriptive Statistics applied in science. It is expected that this book covers all the needs of the students to reach the fundamentals of Descriptive Statistics and to reach the interest and motivation of the students for this topic.