As you might guess from the spelling, Harklinikken is a Danish brand that was developed by doctors working in a (surprisingly chic) hair restoration clinic in Copenhagen. While the star of the line is the "Extract" that's created for each patient after a consultation, the Harklinikken BCB Shampoo, $29, is also incredibly important. It's a deep cleansing shampoo with an unusual, almost watery consistency and herbal scent. It's packed with botanical ingredients, the key being burdock root, which helps regulate sebum production and, in turn, hair growth.

When the folks at Strivectin formulated StrivectinHair Ultimate Restore Shampoo for Damaged or Thinning Hair, $25, they borrowed science from their skin care products. The key ingredient in the shampoo is NIA-114, a special type of niacinamide that nourishes the scalp, allowing it to produce healthier hair.

Recognizing that thinning hair is also generally fine and brittle, they added a strengthening complex made of sweet almond protein, soy and keratin to the formula. It reinforces each hair strand, helping it stand up to styling damage.

Viviscal is primarily known for its hair growth supplements, which support healthy hair from the inside out. And the new Viviscal Gorgeous Growth Densifying Shampoo, $19.99, contains similar ingredients yet works on the hair and scalp externally. Thanks to a special blend of pea sprouts and grape seeds, it encourages hair to grow thicker. Meanwhile, biotin, keratin and zinc nourish each strand.

As we age, our hair changes, and thinning is just one of the possible shifts. The MIT scientists who created Living Proof turned their attention to aging hair and developed Living Proof Timeless Shampoo, $28, to reverse and fight all signs of hair aging. In addition to Living Proof's signature ingredient (OFPMA -- which balances moisture and repels dirt and oil), this shampoo contains amino acids to strengthen strands and ceramide-like lipids to seal hair cuticles, which helps prevents damage and makes hair look fuller and healthier.

Hairmax Density Shampoo, $20, balances scalp oils and deeply cleans hair, but it's the special blend of DHT blockers, antioxidants and botanicals that makes this shampoo special. DHT (dihydrotestosterone) is a hormonal byproduct that shrinks hair follicles and contributes to baldness and hair loss, and there's scientific evidence that shows that topical treatments -- like the ingredients in this shampoo -- may stop DHT before it does damage.