During the first couple years of our career as data scientists, we were bewildered by all kinds of data science hype. There is a lack of definition of many basic terminologies such as “Big Data” and “Data Science.” How big is big? If someone ran into you asked what data science was all about, what would you tell them? What is the difference between the sexy role “Data Scientist” and the traditional “Data Analyst”? How suddenly came all kinds of machine algorithms? All those struck us as confusing and vague as real-world data scientists! But we always felt that there was something real there. After applying data science for many years, we explored it more and had a much better idea about data science. And this book is our endeavor to make data science to a more legitimate field.