Often it's your friend for life, your partner-in-crime, your sister-from-another-mister (or your actual sister, or your brother from another mother) and somebody you know you want by your side as you take the plunge - be that with your dress, or your vows.
But sometimes, the person you thought would be the ideal candidate for taking on the 'duties' of a chief bridesmaid turns out to be a bit of a dud. Maybe you've found she's not so fond of shopping for a gown once you've hit the twelfth bridal boutique on the list, or perhaps it's her party planning that's somewhat lacking.
Perhaps she's not into the wedding at all, and after yet another broken date and missed phonecall, you've found that you're now avoiding each other completely, which is exactly where one anxious bride-to-be has found herself with her maid of honour ahead of her big day.
"I'm getting married July 2019, the dates been set for nearly a year, venue booked since September, I've been engaged since 2016," the worried bride told Mumsnet users before explaining why she has found herself at 'a total loss' with her maid of honour.
"Last year I was dress searching, planned all of them way in advance and with her and she knew how much I wanted her there, sentimentally. She forgot the first time."
While they eventually 'moved on' from the incident - despite the bride thinking that shopping for her wedding dress would have been a 'huge deal' for her friend - her maid of honour continued to miss more dress shopping dates.
She went on to explain that her friend had only ever spoken about hen plans by complaining about how difficult planning one seems, and that she had tried to help her along with the event.
"I told her what I wanted (not a lot) and even sent her links online and who to contact etc and she said that was great. But she hasn't spoken about it since and I honestly think she hasn't thought twice. It's frustrating because she should be excited and sorting things as most of my other close friends would be? It just seems like she's not bothered."
Two years after her engagement and fourteen months ahead of the day, the bride revealed that she's now 'at a loss' with her maid of honour, who seems to have lost interest in the day and pleaded for help from fellow posters, who weren't exactly understanding of her predicament.
"It's your wedding day - not hers," said one user. "Don't expect other people to be as excited especially with such a long engagement. Your wedding is about your commitment to your partner, not hen do's or dress fittings. Focus on that instead!"
"I’m majorly into weddings and had a big white one myself, but didn’t expect anyone to come wedding dress shopping or wedding fayres with me," said another, but added that it was: "not unreasonable to expect her to come good if she has promised to organise a hen though."
"You got engaged two years ago and there's still another 14 months til the wedding?" one user asked, adding: "She's probably bored sh**less of it all."
"I understand you are excited," EthelHornsby said, "but just because she’s an old friend doesn’t mean she has to be - it’s YOUR wedding. Perhaps you should have picked a friend with the same outlook as you."
She continued; "When I got married, back in the dark ages, bridesmaids were expected to turn up on the day, help the bride get ready, and generally muck in to make sure everything went smoothly. Months of shopping, drinking and organising beforehand were not part of it - people have their own lives you know."