Log P, a measure of molecular hydrophobicity, is one of the most significant descriptors in the field of QSAR analysis and rational drug design. It wields an important role in ADMET phenomena and serves as a key predictor of the environmental fate of most chemicals. Log P values can be measured experimentally or predicted from molecular structure. In this chapter, a synopsis of various experimental approaches for the measurement of log P values that run the gamut from the highly reliable and traditional shake-flask method to automated and rapid methodologies is provided. Computational methods that are utilized for rapid and generally accurate log P calculations are also discussed. Since log D (apparent log P) is related to log P via the ionization constant (pKa), common methods for the prediction of pKa values are included. Numerous software programs are now available for the calculation of log P and pKa values and some commonly used ones are listed. Various applications of log P are exemplified by their use in the delineation of various QSAR models pertaining to absorption, metabolism and toxicity phenomena.