French citizenship by descent is not something you apply for. If French nationality passed to you through an unbroken chain of French parents, you are already French, and what you need is proof: a certificat de nationalité française.
The chain is easy to break. The 50-year rule can rule out families who have lived abroad for generations without keeping ties to France, and claims through a mother before 1973 have their own limits. Other routes exist, but they mostly depend on living in France.
Seven guides covering who qualifies, the 50-year rule, grandparent and great-grandparent claims, the certificat de nationalité and the documents you need.
First steps
I
Start Here
Checking your family line
II
Does the Chain Reach You?
Making your claim
III