This document discusses key metrics for Software as a Service (SaaS) businesses including Monthly Recurring Revenue (MRR), churn rate, customer lifetime value (LTV), and others. It defines each metric, how it is calculated, and why it is important to measure. The document advocates measuring these metrics to better understand customer acquisition, retention, and revenue over time in order to improve the business. It provides examples of how leading SaaS companies use these metrics to manage and grow their subscriptions.