Bioanalytical methods employed for the quantitative determination of drugs and their metabolites in biological matrix play a significant role in evaluation and interpretation of bioavailability, bioequivalence and pharmacokinetics data. Because of the introduction of a lot of drugs in the market every year, it becomes necessary to develop newer analytical methods for such drugs. Method development can take a number of forms. At one extreme, it involves adapting an existing method or making minor changes so as to make it suitable for the new application, as for developing a method for the estimation of drugs using a complex analytical technique like HPLC. After the development, there is a need of method validation. Method validation is defined as the process of proving that an analytical technique is acceptable for the intended use and this is an important requirement for analytical purpose. Validation is done according to the guidelines of ICH & FDA. Here, in this book, I have discussed method development and the various parameters used for method validation, namely accuracy, precision, LOD,LOQ, specificity, ruggedness, and range, stability, recovery, matrix effect.