user14680: I read this comment:

You could consider running a pruning node, which is practically the
  same as a full node (including security), except it throws away some
  of the old history. Version 0.11 will make that possible

But I can't find details of how to run bitcoind in pruning mode. Questions on this site seem not to be recent enough to yield answers, and I also don't see a pruning mode in the bitcoind parameters.
Any insights into this?

user35045: there was a followup on what a pruning node could actually be good for. Well, one answer would be shop systems who do not want/cannot rely on third party APIs like blockchain.info for their payment processing. a prune node would allow to be run on say a cheap vps with low space, while checking mempool for incoming transactions to implement payments. it would also allow checking/signing messages using the clients rpc calls, and to a limited degree it could allow checking the balances of addresses. I personally think(and I proposed intelligent pruning as I call it back in 2013) bitcoin devs implemented it wrongly. My implementation suggestion back then was to prune coins from the blockchain that had already been spent. Current behavior is to verify coins in transactions back to coinbase(aka when they were mined), while it should be sufficient to keep the last say 120 blocks plus every unspent transaction. Since the client verifies all blocks it gets, it could rely on those coins to be verified, yet it wouldnt need to know any previous transactions the coins went through.

user5757: There are more details in the release notes.

To enable block pruning set prune=<N> on the command line or in bitcoin.conf, where N is the number of MiB to allot for raw block & undo data.
A value of 0 disables pruning. The minimal value above 0 is 550.

Where 550 is the size in MB you want the node to occupy in your system.
The bitcoin.conf is in the Bitcoin data directory, you can create a text file by the name there, if there isn't a bitcoin.conf yet.

user5292: By setting the -prune option.

-prune=
         Reduce storage requirements by enabling pruning (deleting) of old
         blocks. This allows the pruneblockchain RPC to be called to
         delete specific blocks, and enables automatic pruning of old
         blocks if a target size in MiB is provided. This mode is
         incompatible with -txindex and -rescan. Warning: Reverting this
         setting requires re-downloading the entire blockchain. (default:
         0 = disable pruning blocks, 1 = allow manual pruning via RPC,
  550 = automatically prune block files to stay under the
         specified target size in MiB)

The feature was originally introduced in Bitcoin Core 0.11 (see release notes).
As of version 0.12 pruning can be used while running a wallet. But note the -txindex and -rescan caveats.
Also note that the specified MiB will only include the block files and some undo space. Besides that you will also need some disk space for the chain state and block index databases.

14680: <p>I read <a href="https://bitcoin.stackexchange.com/questions/37494/blockchain-over-36gb-and-causing-disk-space-issues/37495#comment43927_37495">this comment</a>:</p>

<blockquote>
  <p>You could consider running a pruning node, which is practically the
  same as a full node (including security), except it throws away some
  of the old history. Version 0.11 will make that possible</p>
</blockquote>

<p>But I can't find details of how to run <code>bitcoind</code> in pruning mode. Questions on <a href="https://bitcoin.stackexchange.com/questions/19920/how-explicitly-can-the-blockchain-be-pruned">this</a> <a href="https://bitcoin.stackexchange.com/questions/21126/will-old-accepted-blocks-ever-be-removed">site</a> seem not to be recent enough to yield answers, and I also don't see a pruning mode in the <a href="https://en.bitcoin.it/wiki/Running_Bitcoin">bitcoind parameters</a>.</p>

<p>Any insights into this?</p>
 35045: <p>there was a followup on what a pruning node could actually be good for. Well, one answer would be shop systems who do not want/cannot rely on third party APIs like blockchain.info for their payment processing. a prune node would allow to be run on say a cheap vps with low space, while checking mempool for incoming transactions to implement payments. it would also allow checking/signing messages using the clients rpc calls, and to a limited degree it could allow checking the balances of addresses. I personally think(and I proposed intelligent pruning as I call it back in 2013) bitcoin devs implemented it wrongly. My implementation suggestion back then was to prune coins from the blockchain that had already been spent. Current behavior is to verify coins in transactions back to coinbase(aka when they were mined), while it should be sufficient to keep the last say 120 blocks plus every unspent transaction. Since the client verifies all blocks it gets, it could rely on those coins to be verified, yet it wouldnt need to know any previous transactions the coins went through.</p>
 5757: <p>There are more details in the <a href="https://github.com/bitcoin/bitcoin/blob/v0.12.0/doc/release-notes.md" rel="noreferrer">release notes</a>.</p>
<blockquote>
<p>To enable block pruning set <code>prune=&lt;N&gt;</code> on the command line or in <code>bitcoin.conf</code>, where N is the number of MiB to allot for raw block &amp; undo data.</p>
<p>A value of 0 disables pruning. The minimal value above 0 is 550.</p>
</blockquote>
<p>Where 550 is the size in MB you want the node to occupy in your system.</p>
<p>The <code>bitcoin.conf</code> is in the <a href="https://bitcoin.stackexchange.com/q/4006/5406">Bitcoin data directory</a>, you can create a text file by the name there, if there isn't a <code>bitcoin.conf</code> yet.</p>
 5292: <p>By setting the -prune option.</p>

<blockquote>
  <p>-prune=
         Reduce storage requirements by enabling pruning (deleting) of old
         blocks. This allows the pruneblockchain RPC to be called to
         delete specific blocks, and enables automatic pruning of old
         blocks if a target size in MiB is provided. This mode is
         incompatible with -txindex and -rescan. Warning: Reverting this
         setting requires re-downloading the entire blockchain. (default:
         0 = disable pruning blocks, 1 = allow manual pruning via RPC,
  550 = automatically prune block files to stay under the
         specified target size in MiB)</p>
</blockquote>

<p>The feature was originally introduced in Bitcoin Core 0.11 (see <a href="https://github.com/bitcoin/bitcoin/blob/v0.11.0/doc/release-notes.md#block-file-pruning" rel="noreferrer">release notes</a>).</p>

<p>As of version 0.12 pruning can be used while running a wallet. But note the -txindex and -rescan caveats.</p>

<p>Also note that the specified MiB will only include the block files and some undo space. Besides that you will also need some disk space for the chain state and block index databases.</p>
