The protocol is the detailed plan for your systematic review.  It explains your rationale and methodology, and should be developed right at the beginning. You can refine it as you go on. 

Publishing your protocol in either a register or a journal will help avoid duplication - you don’t want other researchers doing the same review as you.  It also “…reduces the impact of review authors’ biases, promotes transparency of methods and processes ... and allows peer review of the planned methods.” Cochrane Handbook, Part 2, Chapter 1.5

If you need more convincing, here are some further arguments for a registered protocol:

• The PLoS Medicine Editors. (2011). Best practice in systematic reviews: The importance of protocols and registration. PLoS Medicine, 8(2), e1001009. doi:10.1371/journal.pmed.1001009
• Equator Network (2015, January 23). Planning a systematic review? Think protocols [Blog post].

It’s also a good idea to check through protocol registers to see if another group of researchers has already registered a review on your topic. (See the "where" box below).

PRISMA-P – Preferred reporting items for systematic review and meta-analysis protocols

This is a guideline to help authors prepare protocols for planned systematic reviews and meta-analyses that provides them with a minimum set of items to be included in the protocol. Check the PRISMA-P checklist and the PRISMA-P statement.

For more information and a good explanation, read this article:

Shamseer, L., Moher, D., Clarke, M., Ghersi, D., Liberati, A., Petticrew, M., . . . Prisma- P. Group. (2015). Preferred reporting items for systematic review and meta-analysis protocols (PRISMA-P) 2015: elaboration and explanation. BMJ, 350, g7647. doi:10.1136/bmj.g7647

Typically, your systematic review protocol should include:

• Your rationale and objectives, including your PICO breakdown (or the relevant framework you are using).
• Your eligibility criteria (inclusions and exclusions).  “One of the features that distinguishes a systematic review from a narrative review is the pre-specification of criteria for including and excluding studies in the review (eligibility criteria)” Cochrane Handbook, section 3.1 This will again match your PICO elements and might include demographic factors, types of studies or geographical considerations.
• Where you will search (databases and grey literature) and your search strategy
• How you will screen your records, extract and manage and analyse the data.

• PROSPERO - This is “an international database of prospectively registered systematic reviews in health and social care, welfare, public health, education, crime, justice, and international development, where there is a health related outcome.” 

Once you register your protocol on PROSPERO, it will be available on open access through their database. Your protocol will have a unique registration number which can be cited in publications and reports to provide the link between your planned and completed review.  

Find out more here:  Guidance notes for registering a systematic review protocol with PROSPERO (University of York Centre for Reviews and Dissemination).

• Cochrane Library - The Cochrane Database of Systematic Reviews includes protocols of Cochrane Reviews that are planned or in progress. When you search for your topic, Cochrane Protocols are listed as a separate category.
• Joanna Briggs Institute – search for systematic review protocols by selecting them in the Publication Types dropdown menu under the search box.
• Protocols are also published in a range of BMC journals such as Systematic Reviews and Environmental Evidence

Many journals will publish protocols - check the journals in your field. Authors’ instructions will often have guidelines for the level of detail expected for the protocols they publish.