Hello, I have a 1 year and 5 month old border collie. His name is Max and he's one of the most loving and affectionate dogs you could meet. However, he recently has shown signs of being over-protective and aggressive towards strangers and when put in certain situations. Examples of this: A) If I am in my room and a member of my household comes up the stairs if the door is open he will guard the door and watch people who are coming upstairs/past the door. If they get close he will physically go for them at first we thought he was playing but after a while he's started getting more aggressive with this and started nipping/biting them leaving visible teeth marks. Just to emphasize this, this can be both family members and strangers who come upstairs. B) When out walking, he use to be really good off lead and could be trusted but as time has gone on he's become more prone to being over-protective of me towards people who approach/come near me. Be this people talking to me or people who are just simply walking by, this has therefore resulted he is not allowed off lead as much unless in enclosed and none-public areas. He is not the best on the lead so this has resulted in him becoming worst on lead by pulling and barking at people who are passing by. I am not sure how to stop him from being over-protective or doing this behaviours in public and this has resulted in me taking him for less walks as I am not able to control him when out. Resulting probably if not likely his behaviours getting worst at home (See example A). C) Today a family member brought a works man in to the house to inspect the garden and did not tell me. I did not expect Max to react how he did but he went over to the man jumped up on him and bit his arm (he did not do it hard but enough to leave visible teeth marks which is unacceptable; luckily the man understood and said he was not bothered as it didn't hurt but this situation is horrendous to be in as it could have been a child or someone who maybe wasn't as hardened to dogs). I understand that Max may not have known the man and may have seen him as an intruder but with me being present and another family member for him to do this is not a situation I would like to be in where I can not trust my dog or for people to be able to come into the house without the possibility of them being harmed by max. I do not know how to deal with this and do not know how to rectify this behaviour or what it is that is triggering him to do so, so I am able to correct this myself. The point of this post was for if anyone could offer any professional advice on this situation or/and if anyone knows any trainers local to M46 area (UK) who specialise in behaviour with Border collies would be really appreciated.