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+## 1. Introduction to GTFS
+
+GTFS (General Transit Feed Specification) is a data standard developed
+by Google used to describe a public transportation system. Its primary
+purpose was to enable public transit agencies to upload their schedules
+to Google Transit so that users of Google Maps could easily figure out
+which bus, train, ferry or otherwise to catch.
+
+> **A GTFS feed is a ZIP file that contains a series of CSV files that
+list routes, stops and trips in a public transportation system.**
+
+This book examines GTFS in detail, including which data from a public
+transportation system can be represented, how to extract data, and
+explores some more advanced techniques for optimizing and querying data.
+
+The official GTFS specification has been referenced a number of times in
+this book. It is strongly recommended you are familiar with it. You can
+view at the following URL:
+
+<https://developers.google.com/transit/gtfs/reference>
+
+### Structure of a GTFS Feed
+
+A GTFS feed is a series of CSV files, which means that it is trivial to
+include additional files in a feed. Additionally, files required as part
+of the specification can also include additional columns. For this
+reason, feeds from different agencies generally include different levels
+of detail.
+
+***Note:** The files in a GTFS feed are CSV files, but use a file
+extension of `.txt`.*
+
+A GTFS feed can be described as follows:
+
+> **A GTFS feed has one or more routes. Each route (`routes.txt`) has one or
+more trips (`trips.txt`). Each trip visits a series of stops (`stops.txt`)
+at specified times (`stop_times.txt`). Trips and stop times only contain
+time of day information; the calendar is used to determine on which days
+a given trip runs (`calendar.txt` and `calendar_dates.txt`).**
+
+The following chapters cover the main files that are included in all
+GTFS feeds. For each file, the main columns are covered, as well as
+optional columns that can be included. This book also covers some of the
+unofficial columns that some agencies choose to include.
+