What are Topics API and Fledge API? | Articles

In recent years, Internet users have become increasingly concerned about their privacy and the use of their personal data on the Internet. Consequently, some legal measures have been taken and now advertisers and publishers need to adapt the way they advertise to offer an environment that better respects the privacy of users.

On the other hand, to follow the trend of other browsers, Google has announced that they will remove third-party cookies in Chrome by the end of 2024 (deadline already reviewed several times).

As a reminder, what are third-party cookies? They are a small piece of code that is generated and placed on the user's device by websites, different from the one the user is visiting. Third-party cookies are currently mainly used to track users between different websites, create a profile from the information collected and then allow to display personalized ads. Third-party cookies are therefore powerful tools for online marketing. If you want more information about third-party cookies you can read the article here.

Given that Google Chrome currently represents a majority of the traffic on the Internet, it is time for advertisers and publishers to find alternatives for online advertising. In this context, Google has developed the Privacy Sandbox, a collaborative initiative to create technologies that both protect people's privacy online and give companies tools to build successful digital strategies.

The Google Privacy Sandbox revolves around 4 axes:

  • Fight spam and fraud on the web
  • Show relevant content and ads
  • Measure digital ads
  • Strenghten cross-site privacy boundaries

In the second axis, show relevant content and ads, there are 2 projects on which Google is working: Topics API and Fledge API, about which you will find more information below.

Topics API

This new feature will support interest-based targeting and aims to find new potential customers online.

How will it work?

Depending on the websites you visit, the browser will determine a couple of topics that are supposed to represent your interests. For example, the topics that can be assigned to you are “Beauty”, “Running” or “Travel”. For this to be possible, the API will first need to label each website where Google Topics API is enabled to a set of topics.

In more detail, when a user visits a website, the API will share 3 topics, one from each of the last three weeks, with the website and their advertising partners. Then, it will be possible to use these topics as one of the signals for interest-based advertising. For example, if you're promoting beauty products, you'll be able to target users categorized in the "Beauty" topic while remaining relevant and private.

You are probably wondering how Topics API will be more private than the current affinity targeting?

  1. Users will be able to clearly see the topics they are associated with, remove them, or opt out of the Privacy Sandbox APIs entirely.
  2. To limit the amount of data associated with an individual at any given time, topics will be stored for 3 weeks, after which they will be deleted. This means that you are associated with one new topic per week.
  3. There will be a limited set of topics which helps reduce the chance that individuals can be identified based on their unique topics of interest.
  4. The set of topics will be public and manually curated. The list will be developed and maintained in collaboration with the internet ecosystem to allow to provide feedback.

Please note that there is currently a type of targeting called Topics on Google. This targeting is based on the content of the websites where the ads will appear, this is called contextual targeting and has nothing to do with Topics API.

In a nutshell, Topics API will replace our current interests targeting, in a more private way and should not be confused with the Topics targeting (contextual).

Fledge API

Initially, this feature was about finding a way to support remarketing advertising but it's broader than that. Fledge will be a new way to create custom audiences that are not tied to cookies, which goes beyond simple retargeting.

How will it work?

If we take the example of the beauty products advertiser: suppose a user is browsing the internet and looks at a face cream on your website.

You, as the advertiser, can inform his/her browser that you would like to show him/her ads in the future. You can also share information such as the type of ads you want to show and how much you are willing to pay.

The Chrome browser will then place the user in an interest group “face cream”. At this point, the browser knows that the user may be looking for a face cream.

Then, the next time that this user visits a website with ad placements, an algorithm in the browser will run an auction of ads based on the interest group “face cream” and show relevant ads.

The Fledge API therefore enables on-device bidding by the browser to choose relevant ads from websites the user has previously visited.

Important to note that the advertiser is informed whether anyone clicked on the winning ad but they know nothing else about the person other than his/her purported interest in face cream. Moreover, interest groups are controlled by the browser rather than advertisers or ad tech platforms.

We know the difference between Topics API and Fledge API is complicated to understand, but Topics API should be seen as a simplified version of Fledge. Topics API is a way to tap into predefined audiences and Fledge as a way to find and target custom audiences.


It's time to start educating ourselves on the cookieless alternatives that are being developed. Topics API and Fledge API are fairly recent solutions but we will keep an eye on them at Semetis to see what developments are made and test them on our clients as soon as they are available.

publication author olivia lohest
Olivia Lohest

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