What's the plant and where is it? Indoors, outside in the ground, outside in a container? When did you plant it & what was the last thing you did to it & when?

All important questions--there are so many things that can cause yellow leaves, and sometimes only the plant's life history can provide answers. To Beattie's list, I would add:
Do you feed it, and if so, what, how much, and how often?
Do you water it, and if so, how much and how often.
How much light is it getting?

This might be of use ...
http://www.growsonyou.com/question/show/34378

... anthedesia

If you don't know what it is could you post a photo?

anthedesia, a gift in a pot about 4 weeks ago, kept indoors in plenty of light and watered every few days, not swamped. Not yet fed.

Taking care of anthedesia from terratoonie's lead above ...

"High light levels are needed without direct light, any well lit windowsill away from direct sun will be ideal.

Keep compost just moist, with free drainage of bark chips and sphagnum moss. Use tepid SOFT chlorine free (leave water butt one week after filling with tap water to allow chlorine to dissipate, water and spray daily.

When flowering, apply a general liquid fertilizer. Keep tight in pots but re-pot annually.

Temperatures around 64deg F are ideal , a degree or two higher or lower is fine."

Specialist mineral based feeds can be applied. Similar treatment to an orchid??

Many thanks

Sorry Gmartin. For some reason, when it first appeared, I didn't see the "on plant" designation.

Hello Tugbrethil ... I wish that particular bit of info to be in bold red caps; it is too easily missed in its current position. Is this one to flag to the administrators of the web-site?

I think that's because it wasn't there at first, Tugbrethil. I could swear I looked for it when I was mystified by the question in its original state.

I didn't see it either. Its very faint.