After the disastrous first contact with the Klingons in 2223, and the increased tensions with the Romulan Empire, Starfleet secured research funding for what they termed the Starfleet Expansion Program (full project name: Starfleet Expansion and Modernization Program) in 2335, to bring about at least 5 new ship classes (cruiser, scout, destroyer, frigate, corvette) and build at least 20 of each class. Registry numbers were reserved, as was the now common practice, and Starfleet began its research project. The first ships built started rolling out of starbase, featuring a unified technology making upgrades easy and more cost-effective, and engine and other component upgrades much simpler.
The most general purpose ship used by Starfleet, the Cruisers were the most all-purpose type starship and the most well equipped. The Constitution class had an original construction contract for 13 ships, and the Anton class had a contract for 15. The Anton-class would go on to become an excellent research vessel for the more scientifically-minded, while the Constitution class would distinguish itself in both combat and discovery and exploration.
Given the improvements in warp theory and technology, Starfleet fielded two scouts as proof-of-concept that a single warp nacelle could serve a starship, partly for reduced cost, and for simplified maintenance
The Ptolemy class vessel became the premier transport for Starfleet, helping to bring men and materials to build many planet-bound and orbital starbases in many places, including along the neutral zones with the Klingons and Romulans.