Internet censorship can constitute a violation of human rights (such as the right to freedom of expression and the right to access information), yet there is limited transparency of where, why, and how it occurs around the world. Identifying the intentional blocking of sites and services can be challenging. For starters, cases of Internet censorship are easier to notice when they affect services that are commonly used (such as WhatsApp) and receive media coverage, while the blocking of less popular sites and services (such as the blocking of minority group sites) often goes unnoticed. The fact that an Internet service is inaccessible doesn’t necessarily mean that it is intentionally blocked by an Internet Service Provider (ISP), as there are many reasons why a service may be inaccessible. Perhaps that service is hosted on an unreliable server, or its owner is blocking all IP addresses originating from a specific country. Internet censorship can vary from network to network within a country, as different Internet Service Providers (ISPs) often block access to different websites, often as a result of vague government orders. Most censorship techniques are quite subtle, making it hard for the average Internet user to distinguish a case of blocking from a transient network failure, a case of DNS misconfiguration, or other reasons why a service may be inaccessible. There is also limited transparency and oversight over cases of over-blocking and when Internet censorship results in collateral damage. In Indonesia, for example, Vimeo and Reddit were found to be blocked on a mobile network, even though their ban had been lifted more than 2 years ago. In Egypt, the blocking of a media website resulted in the collateral blocking of thousands of other websites that were hosted on the same Content Delivery Network (CDN). In order to monitor all cases of Internet censorship and ensure that they are lawful, it is necessary to measure networks to detect such cases. The Open Observatory of Network Interference (OONI) has therefore built free and open source software that anyone can run to measure networks and collect evidence of Internet censorship. Through this session, participants will learn how to investigate Internet censorship through the use of OONI’s censorship measurement tools and open data. As a result, they will be able to use network measurements as evidence of Internet censorship and support their advocacy and policy efforts.