Battery pack design for transportation is a rapidly evolving and fascinating topic at the intersection of electrical, mechanical, and software engineering. We will discuss how to translate vehicle level requirements such as vehicle weight, speed, drag, range, and recharge time into battery pack specifications such as voltage, energy (kWh), and power (kW). We will then discuss the typical design of a lithium ion battery pack including cells, packaging, thermal management, battery management, and safety and look at a few examples from scooters, motorcycles, cars, and airplanes. Next we will look at lithium ion battery pack safety and various strategies to mitigate the failure modes. Finally, we will review the cutting edge of battery pack design and a few potentially breakthrough technologies.