The barometer remains the most important tool for evaluating and predicting the weather. This book explains why knowing accurate values of the atmospheric pressure can improve this process and benefit all applications. Ways to evaluate and calibrate aneroid and electronic barometers using readily available data by Internet or telephone are clearly described. Tactical applications to marine navigation are covered. The book also includes worldwide average monthly pressures and their standard deviations. With the exception of trained meteorological crews, barometers in the past were used at sea primarily to learn if the pressure was going up or down, and whether it was doing this rapidly or slowly. And from this we made an educated guess of a likely consequence, usually in conjunction with other observations. Outside of professional marine use, little effort was made to know the true pressure for several reasons. We did not know what to do with it if we had it, and more often we could not get it if we wanted it, because most barometers were not calibrated. This has changed. We now have many valuable applications of accurate pressure, and we also have affordable ways to get it, and we have convenient ways to prove our instruments work properly. The main goal of this book is to show how all this comes about, and to give more insight into this fascinating, venerable instrument that has been with us for 150 years. Another goal is to introduce users to the new breed of electronic devices that are in the process of revolutionizing how we do weather at sea. We also re-introduce mariners to several classic aneroid instruments that are highly accurate and tested at sea for over 70 years, and why we benifit from having both a calibrated aneroid and an electronic barometer on board.