diff --git a/book/source/09-verification.md b/book/source/09-verification.md
index 059c48d..8f638e3 100644
--- a/book/source/09-verification.md
+++ b/book/source/09-verification.md
@@ -32,6 +32,18 @@ This is especially the case with signatures created by dedicated signing subkeys
 * **Revocation**:
   Lastly, signatures can be invalidated by revocations.
 
+### Well-formedness of signatures
+There is a number of criteria, that a signature must fulfill in order to be considered well-formed:
+
+- Each signature MUST have a signature creation time subpacket in its hashed subpacket area. A signature with only a unhashed creation time - or none at all - is not well-formed.
+- The signature cannot be older than the key that issued it.
+- Analogous, a signature with a creation time in the future needs to be rejected as well.
+- A well-formed signature needs to carry an Issuer Fingerprint subpacket, or an Issuer KeyID subpacket.
+It is generally recommended to place those in the hashed area of the signature, but a receiving implementation may also accept signatures which only contain unhashed copies of these subpackets.
+- A signature disqualifies as well-formed, if it contains subpackets unknown to the implementation, which are marked as critical.
+Unknown subpackets which are not marked as critical do not have an effect on whether the signature is well-formed.
+- The same applies to notations. Critical, unknown notations result render the signature malformed.
+
 (temporal-validity)=
 ### Temporal validity