Experts recommend that you should set up the minimal hierarchical folder structure that is logical for you, and use tags for all other categories. For example, Kerry Gleeson's classic book *The Personal Efficiency Program* (1994, 4th edition 2009) recommends the following three folders: 1. Working, 2. Reference, 3. Archives. "Working" would contain files related to projects you have not yet finished (organized into subfolders by project); "Archives" would contain finished projects (perhaps organized into subfolders by year or month); "Reference" would contain files that are unrelated to particular projects. David Allen's classic book *Getting Things Done* (2001) recommends similar categories. But these are just examples; use whatever folder structure seems right for you. You can manage tags in many ways, which I will describe in detail below: 1. in the Finder, 2. in Spotlight searches and Smart Folders, 3. in third-party apps, 4. on the command line, 5. in scripts, 6. in *Save* dialog boxes, and 7. in many apps, by clicking on the title bar of a document window to see and edit a list of tags for that document. **In the Finder**, make sure you have the *Edit Tags* button enabled in the toolbar of Finder windows (it is enabled by default, but if you disabled it you can enable it again from the Finder menu *View > Customize Toolbar...*) and the *Tags* list enabled in the sidebar of Finder windows (it too is enabled by default, but if you disabled it you can enable it again from the Finder menu *Finder > Preferences... > Sidebar*). You can customize the list of tags in the sidebar from the Finder menu *Finder > Preferences... > Tags*. In the Finder, you can select file(s) and then add or remove tags by using the *Edit Tags* button or the *Get Info* window (command + I) or the *Inspector* modal window (option + command + I). If you click on your computer in the sidebar of a Finder window and then use the search field, you can also edit the tags of any email that appears in the search results. You can search by tags **in the search field** in the toolbar of Finder windows: just start typing the name of a tag, and a menu will drop down with a list of tags: click on the desired tag to finish the search. (Then click on the *Save* button if you want to save the search as a Smart Folder.) You can search for tags **in Spotlight searches** by typing the prefix `tag:` and then the tag name. In both of these search fields, you can also search for multiple tags at once and use the Boolean operators AND, OR, and NOT (or the minus sign `–`, which means NOT) to narrow search results: for example, `tag:trip NOT tag:france` is equivalent to `tag:trip -tag:france`. The Finder is not always the best way to browse by tags. [Some people have called tags "Finder tags"](https://www.google.com/search?q=%22Finder+tags%22)—but this phrase is a misnomer because tags are an *extended attribute* in the filesystem and are not limited only to the Finder! You can also tag files and browse by tags **in third-party apps** such as Fresh and Leap from [Ironic Software](http://www.ironicsoftware.com/). I find [Leap](http://www.ironicsoftware.com/leap/) to be an especially excellent way to manage tags, thanks to its tag cloud and many ways of viewing files that are not available in the Finder. You can also index your files in the information-management app [DEVONthink](http://www.devontechnologies.com/products/devonthink/overview.html), which reads and writes standard tags but also allows you to organize tags into a hierarchy, which allows for what librarians call faceted classification. The easiest way to use tags **on the command line** is to install James Berry's command-line tool `tag`, which can be installed with either of two package managers: Homebrew (`brew install tag`) or MacPorts (`sudo port install tag`). After installing `tag`, you can use it to search for tags and to tag files on the command line; for more information, consult `tag --help` or `man tag`. You can also use `tag` **in scripts**, either stand-alone shell scripts or as part of an Automator workflow. Here is [the tagging philosophy of software developer and tag enthusiast Brett Terpstra](http://brettterpstra.com/2011/02/22/on-sorting-tagging-and-other-nerdery/): > The beauty of tagging, as you may know, is that you can easily assign multiple categorizations and topics to each item, rather than just having them exist at one location which defines it as a type or part of a static collection. I still use the shallow hierarchy of folders that drill down to individual projects and topics, so it's not a "one pile" deal. I couldn't function like that; it's difficult to weed, and if metadata is lost, so is the file, essentially. I use folders to maintain filesystem sanity. I use tags and other metadata to maintain *my* sanity. > I also use tags on iCal events, emails, web sites and other elements of my system. They're easy to filter in searches and allow me to make a project "folder" that contains more types of data than Finder would normally make convenient. > [...] > The secret, which I've learned from a few years of doing this, is to tag intelligently, sparingly and consistently. Being liberal with tags like "important" or "flagged" just means more upkeep and reviews that have to be done to keep the tags relevant. While tags like "neat" or "supercool" seem silly to me, I use one "inspiration" tag on anything that I'm saving just because it's, well, inspirational. I have a Smart Folder in Finder that pulls up everything with that tag, and then I can filter the files based on date, type, etc. If I add additional tags, they are pretty general, such as "color," "webdesign," or "productivity." These are big buckets I can dive into when I need an idea, or a kick in the pants. The tags on project and research files tend to be more specific, and I can drill down to precise sets of files by typing a few words in Tags or HoudahSpot. > To keep the cruft down, I use tags which aren't already defined by other metadata like the creation or modified date, the filetype, the filename, etc. I do tag files with the name of their parent project or topic, even though it's often redundant to the folder hierarchy. This allows me to tie projects together across multiple folders, disks or even servers. Sometimes a file is more strongly tied to a topic than it is to an individual project. Say it's a general reference item or a stock photo that I gathered while working on a specific project, but that I know I'll need again. I put it into a reference folder or photo collection, and tag it with the related project. It's easy to view it in the context of the project with a tag search, but part of a larger collection of similar items at the same time. > When in doubt, I usually choose tags based on the first thought that pops into my head. Chances are, when I'm looking for it again in a year, I'll have the same thought. It doesn't always work, but I've had a pretty good success rate over the last couple of years. As often as possible I try to reuse tags; a tag that reveals one or two files is a useless tag. My "Yearly Review" includes weeding out tags that have very few associated files, or that could be merged into more common tags. Keep in mind that you may lose your tags if you transfer files to a filesystem (notably, Windows) that doesn't support tags, or if you use a version control system like Git that doesn't support tags. Some important reading on this topic includes: * Brett Terpstra's blog post ["How to lose your tags"](http://brettterpstra.com/2014/07/04/how-to-lose-your-tags/) * The Ask Different question ["Are tags portable, and if so how?"](https://apple.stackexchange.com/questions/122251) **This is my first answer on Ask Different, so I don't yet have the 10 reputation required to include more live links; if you upvote this answer so that my reputation increases, I can add more live links.**