In order to provide encrypted traffic to users, a site must first apply for a certificate from a trusted Certificate Authority (CA). This certificate is then presented to the browser to authenticate the site the user is trying to access.
In recent years, due to structural flaws in the HTTPS certificate system, certificates and issuing CAs have proven vulnerable to compromise and manipulation. Google's Certificate Transparency project aims to safeguard the certificate issuance process by providing an open framework for monitoring and auditing HTTPS certificates.
Google encourages all CAs to write the certificates they issue to publicly verifiable, append-only, tamper-proof logs. In the future, Chrome and other browsers may decide not to accept certificates that have not been written to such logs.
As of 2015-01-01T12:34, there have been 32,341,621 entries made to the set of Certificate Transparency logs that Google monitors.
More information about the Certificate Transparency project: https://www.certificate-transparency.org/
Issuing Certificate Authorities