Absolutely no nudity. This is a children's animated show anyway.

In one episode, Astrid is briefly seen kissing Hiccup.

In one episode, Tuffnut makes a comment about Ruffnut's butt.

In one episode, Fishlegs attempts to kiss Astrid. But Astrid makes a remark that nobody's gonna kiss her lips forever.

In one of the third season's episodes, Ruff has lost a contest, and thus had to swap her clothes to her twin brother Tuff. This is only talked about, not shown.

In one episode, it is implied Snotlout is peeing, as he is seen behind some bushes and asks if "they could have some privacy".

This animated series is mainly about dragons. In several episodes, at least one dragon tend to attack humans and other dragons. Sometimes humans fight dragons.

Dragons can attack by breathing fire, spit acid, go on direct attack and bite, use their tails, etc.

Weapons like sword, axes and spears are often seen. In some episodes, several people use at least one of the weapons either at each other or a certain dragon, depending on the episode.

Words like "kill" or "get killed" can be heard sometimes being uttered by any character. "Blood" can rarely be heard in a few episodes. Also, references to having stuff like a head or the legs to be cut off are mentioned in some episodes.

Characters tend to fall down from heighs, hit on each other, get hit by objects, get attacked by (even their own) dragons, etc. However, they appear to be completely fine in most circumstances, especially in scenes made for a laugh.

Tuffnut and Ruffnut tend to hit on each other constantly many times they appear on screen. These scenes are mostly made for a laugh.

Some house get burned or simply get destroyed in a few episodes, depending on circumstances.

In some episodes, humans attack each other. These scenes are mostly pretty mild.

In one episode there is a burning forest.

In one episode of the third season, a small group of people is mentioned to have been "wiped out" by a dragon in the middle of a snowstorm. Also, a man's foot is set in fire, and a man vomits constantly (vomit not directly seen). The latter mentioned scenes are more likely made for a laugh.

A baby is about to get hit by a wagon of apples, until a hypnotised Fishlegs saves it.

In one episode, a horde of fireworms is about to burn down several places in an island.

A huge wave is headed toward an island in one episode.

Words like "stupid" and "idiot" can be heart a few times in some episodes.

Some dragons may look scary, and can attack without warning. Sometimes, characters are put into various but dangerous situations. However, since this is a TV-PG rated children's animated show, children should have no problem watching it.