27th (Home Counties) Anti-Aircraft Brigade (27 AA Bde) was an Air Defence formation of the British Army in the Second World War that served in The Blitz and later converted to infantry.

3rd AD Bde's units were initially based at Lytton Grove, Putney, taking over buildings previously used by the City of London Yeomanry (Rough Riders).

As part of this reorganisation, 26th (London) Bde assumed command of all the gun and searchlight units of the two former brigades, while the 27th was reformed as 27th (Home Counties) Anti-Aircraft Group, based at RAF Kenley, to command new AA units in the South London suburbs.

[5][6][7][8] The AA Groups took the more usual formation title of Brigades on 1 January 1939 after the Royal Artillery replaced its traditional unit designation 'Brigade' by the modern 'Regiment'.

[6] During the period of tension leading to the Munich crisis and eventually the outbreak of the Second World War, the Territorial Army grew enormously, and existing TA infantry battalions continued to be converted to AA regiments.

The number of divisions and brigades was expanded, and the whole AA defence of the United Kingdom was taken over by Anti-Aircraft Command on 1 April 1939.

[24][25] Over the two years following the end of the Blitz, the brigade had the following changes in composition:[26][27][28][29] Increasingly, HAA and support units were 'Mixed', indicating that the operational personnel included women of the Auxiliary Territorial Service (ATS).

The AA defences of Southern England were severely tested in the summer of 1942 by the Luftwaffe's 'hit-and-run' attacks along the South Coast, and there was much reorganisation, accounting for some of the turnover of units listed earlier.

[36] By the summer of 1943 the brigade had the following composition:[37] From September 1943, 33rd S/L Rgt carried out experiments for AA Command on new combinations of S/Ls and searchlight control radar (SLC).

[42] At the same time the Luftwaffe was suffering from such shortages of pilots, aircraft and fuel that serious aerial attacks on the United Kingdom could be discounted.