My only bike is a cross bike, and I plan to do several very hilly endurance rides this year, including the Ironman in September. I'd like to add some lower gearing to help make the hills a bit easier. Currently, I have a mostly Ultegra SL drivetrain, with FSA 36/46T crankset, and Ultegra 11-25T cassette. I thought about swapping out the 36T ring for a 34T ring, and/or swapping the rear cassette for an 11-27T. Are there any compatibility issues to be aware of? Any other options I haven't listed?

The steps you mentioned would be the first to consider, and will not introduce any compatibility issues except that you might need a longer chain.

For more serious hill climbing the next steps are a triple crank (lots involved there) and a cassette with even larger cogs like 32 or 34 teeth (new rear derailer.)

I would figure out what gears you want/need using a gear calculator and decide based on that. Just me guessing, a 34 ring combined with a 28T largest cog on the cassette should work with no other modifications and should give you all the gear you'll need.

A compact crank up front with 50/34T would give you slightly lower and slightly higher gearing. You can also get an 11-28 cassette, too. Note, some Ultegra derailleurs have a max of 27T and the newer one can accomodate 28T. If you can read the derailleur model number off the back, I can tell you if it can handle 27 or 28T max. (I just went thru this a couple of weeks ago, bought a new cassette only to find I need a matching derailleur.

If you go with a "mountain" cassette, then a long cage derailleur would be necessary.

Going to a triple crank would involve at minimum: new front shifter/brifter, new crankset, new rear derailleur = no fun.

Either one or both is possible. The 6600 (10 speed Ultegra SL) front derailleur is rated at 16t and you're currently only at 10t. The chainwrap on your rear derailleur (6600 SS) is 29T, currently it's taking up max 24t. Adding two teeth to the chainring and two teeth to the low cog would increase that to 28t. As I said your front derailleur is rated at 16t and and the reduction of 2 teeth (to 34t) you would only be at 12t. If you want to just do one, bang for the buck is 27T cog (max for your derailleur). Currently your lowest gear ratio is 1.44 to 1. Bumping up to a 27T cog would lower the gear ratio to 1.33 to 1. Just reducing the front chainring would reduce your ratio to 1.36 to 1; doing both would drop it down to 1.25 to one.

As you increase the spread of your gearing, keep in mind that enlarging the cassette may necessitate a longer chain, but that isn't really an issue because IMO you should always start new cassettes with new chains.

Also keep track of the total tooth differential, big/big - small/small, and compare that to the RD capacity. There's usually a bit of wiggle room, but you have to be cautious when running RDs above rated capacity. If it's a short or medium term thing, you might live with a chain too long to run the inner with the smallest few rear sprockets sparing you the need to buy a new RD.

I like the Campag triple, mine a 50,40, 24 set.135/72bcd
have similar ratio set on my touring rig, 110/72.

10 & 16t differences.. granny like half the size of the big ring..

Changing from your current 36/25 to 34/28 will lower your low gear by 18% which is significant. Whether this will be enough is your decision. The next steps will be a lot more expensive as they involve a MTB cassette ( new rear derailleur and chain at a minimum) or a triple cank (new crank, bottom bracket, front derailleur and possibly a new front shifter).

Run the Sheldon Brown gear calculator before you spend money. +1 18% is quite a bit. Whether it is enough, try it out on the biggest, meanest hill you can find.

Wow, thanks for the great responses. I checked out the gear calculator, and I'm going ahead with the 34T chain ring and 27T cog. I let you know how it turns out.

I'm looking at these chain rings and I want to make sure they're compatible with my existing 36/46T crank set:

FSA Pro Road S9/C10 34t 110mm Black Chainring (use w/50t) - FSA

FSA Pro Road Chainring - 110mm Black, 110x36T S10

I know my bolt circle is 110mm, but the first link says "use w/50T" (I have 46T). The second link says "S10" in the title - is that 10 speed? Second link says "S9/C10", which I don't understand.

If you want slightly lower gearing you can probably run a 12-30 cassette in your Ultegra derailleur OK. My cross bike has a 50/34 crankset and I have the 12-30 cassette for long trail rides using 5600 GS derailleurs (previously had a 6600 GS derailleur that broke in a crash), and it works fine. If your derailleur is short cage it may not have enough chain takeup as you approach the small/small. I run 11-28 for hilly road riding, 12-27 for actual cross, and 12-30 for longer trail rides. The 50t chainring isn't super awesome for actual cross, but oh well.

The ramps and pins of the chainrings are probably designed as sets and will shift slightly faster if paired as recommended. I'd assume S9/C10 would be Shimano 9, Campy 10? Don't know.

That's interesting. I may try the 12-30 cassette, and not even mess with the chainring. I don't think I'd ever be in the small-small anyway.

edit: If I don't change the chainring, can I jump to a SRAM 12-32, and have a decent chance of success? Using onespeedbiker's number above, that would put it one tooth over the maximum chainwrap capacity of the RD.

Your max cog size "recommended" for that RDER is 27 or 28.
32 is MOST likely too much.

Ok, now I'm back to deciding between these options:

1) 12-30T Cassette, keep 36T chainring
2) 12-28T Cassette, replace with 34T chainring

Interestingly, they both yield the same exact gear ratio change. Any preference on one or the other? The 30T Harris cassette weighs 70% more than the 28T Ultegra cassette, but is half the price.

Since the MAX "recommended" cassette is a 27 or 28, I know which way I'd go.
Even the 30T cog MIGHT be problematic.

This bike was set up for only one reason--to climb hills.

It has a 48/36/26 crankset and an 11-34 cassette. The most bang came from the 11-34 cluster.

P.S. It also does relatively well on the flats.