Q: Is there a way of putting old DVD footage into imovie

If you're referring to commercial and hence copy-protected video DVDs, breaking the copy protection, which would be necessary to copy the videos from the DVD and convert them, is illegal in the US (and other countries) and hence Apple explicitly forbids such posts. We're therefore not allowed to discuss it here.

If these are home movies that you or someone has burned to DVD and hence are not copy-protected, Handbrake (freeware) is one of the more popular converters. There are also commercial converters such as DVDxDV:

http://www.dvdxdv.com/NewFolderLookSite/Products/DVDxDV.overview.htm

Regards.

You need to convert the VOB files in the TS-Folder of the DVD back to DV which iMovie is designed to handle. For that you need mpegStreamclip:

http://www.squared5.com/svideo/mpeg-streamclip-mac.html

which is free, but you must also have the  Apple mpeg2 plugin :

http://store.apple.com/us/product/D2187Z/A/quicktime-mpeg-2-playback-component-f or-mac-os-x

(unless you are running Lion in which case see below))

which is a mere $20.

Another possibility is to use DVDxDV:

http://www.dvdxdv.com/NewFolderLookSite/Products/DVDxDV.overview.htm

which costs $25.

For the benefit of others who may read this thread:

Obviously the foregoing only applies to DVDs you have made yourself, or other home-made DVDs that have been given to you. It will NOT work on copy-protected commercial DVDs, which in any case would be illegal.

And from the TOU of these forums:

Keep within the Law

1. No material may be submitted that is intended to promote or commit an illegal act.
3. Do not submit software or descriptions of processes that break or otherwise ‘work around’ digital rights management software or hardware. This includes conversations about ‘ripping’ DVDs or working around FairPlay software used on the iTunes Store.

If you are running Lion:

From the MPEG Streamclip homepage

The installer of the MPEG-2 Playback Component may refuse to install the component in Lion. Apple states the component is unnecessary in Lion onwards, however MPEG Streamclip still needs it. See this:

http://support.apple.com/kb/HT3381

To install the component in Lion, please download MPEG Streamclip 1.9.3b7 beta above; inside the disk image you will find the Utility MPEG2 Component Lion: use it to install the MPEG-2 Playback Component in Lion. The original installer's disk image (QuickTimeMPEG2.dmg) is required.

The current versions of MPEG Streamclip cannot take advantage of the built-in MPEG-2 functionality of Lion. For MPEG-2 files you still need to install the QuickTime MPEG-2 Playback Component, which is not preinstalled in Lion. You don't have to install QuickTime 7.

Is there any software that i can use to decode my DVD's so that imovie can read them?

To add to what others have already said, iMovie requires you convert the MPEG-2/AC3 or MPEG-2/PCM DVD content to a video edit compatible compression format. Many codecs woud be available here depending on your system's specific QT codec component configuration. The advantage of using the free MPEG Streamclip app is the fact that you can convert to any available compression format that is iMovie compatible. This can include DV, AIC, MJPEG, MPEG-4, Motion PNG, H.264, etc.video with uncompressed, AIFF, AAC, or other forms of audio. However, if you do not wish to invest in the QT MPEG-2 Playback component for use with MPEG Streamclip, other free apps (like HandBrake) are also available. The disadvantage here is usually a limited choice of output formats—normally just MPEG-4/AAC or H.264/AAC compression combinations.

So what workflow/compression format would be best for you? This normally depends on your specific goals and system capabilities. For instance, if you are trying to maintain higer quality and have a lot of storage space available for the storage of large intermediate files, then a video compression format like DV or AIC might be your best choice. If quality is of lesser import and you only have moderate amounts of storage space available, then motion PNG or JPEG may be your best choice as an intermediate compression format. Lastly, if all your are doing is sharing files to the internet or via email and don't really have much space available for storing intermediate files, then a data rate limited MPEG-4 or H.264 intermediate format may be your best option. In short, your spercific workflow here is normally dictated by both your goals and you system's configuration/capabilities. There are many different software apps available—both free and commercial—which should serve your needs once you can actually define what your needs and goals are.